I cannot access the iTunes store. I get an itunes store: error -50

iTunes for Windows iTunes can contact the iPhone iPad or iPod software...
If you can access your device's settings menu and connect to Wi-Fi, install the latest version of iOS over Wi-Fi using Settings > General > Software Update. If your device is locked with a forgotten passcode or in recovery mode, you will need iTunes to restore the device.
If one of the alert messages above appears when you try to update or restore your iOS device, connect to the Internet with your preferred web browser, and access the iTunes Store.

If you cannot connect to the Internet with your web browser

If you can connect to the Internet, but see an error when accessing the iTunes Store

If the iTunes Store shows a blank white page

If the iTunes Store opens successfully

From http://support.apple.com/kb/TS3297

iTunes Store loads partially or returns "Error 306" or "Error 10054"
Proxies, parental control settings and software, security or filtering software, or a bad iTunes Store cache can cause this.
To address proxies, Remove Internet Options proxy settings and connect to the Internet without a proxy.
To reset iTunes Store cache:

1. In iTunes, choose iTunes > Preferences (Mac) or Edit > Preferences (PC).
2. Click the Advanced tab.
3. Click the "Reset cache" button.
4. Click OK and see if the issue is resolved.

Adjust Parental Controls in iTunes:

1. Open iTunes.
2. Access iTunes preferences:
   1. On a Mac: From the iTunes menu, choose Preferences.
   2. On a Windows PC: From the Edit menu, choose Preferences.
3. Click the Parental Controls tab.
4. Remove restrictions on Parental Controls.

Macafee security center is the culprit.
Open Macafee then: select;
Internet & Network > configure
Firewall > Advanced
Programm Permissions > scroll down to Itunes
Highlight (click on)
Then Grant full access to Itunes.
APPLY then OK

When you select store from Itunes you will need to register with Itunes

First of all what operating system are you using? And most of the time this "access" issue is due to you either not being the administrator of your computer, or your operating system is not allowing access to the device. If it's iTunes, make sure that you have an account with iTunes, the new iTunes requires you to have an account with the iTunes music store before you can do anything with it.

ResolutionClick on the description below that most closely matches your iTunes Store situation:

• I have been unable to connect to the iTunes Store...
  • for less than 24 hours.
  • for more than 24 hours on a Mac.
  • for more than 24 hours on Windows.
• I have never been able to connect to the iTunes Store on this computer.

I have been unable to connect for less than 24 hours Try quitting iTunes and reopening it. If the issue persists, it's possible the store is temporarily unavailable for maintenance. Try again in a few hours. You might want to check the iTunes Store discussion board to see if a connection issue is impacting many other iTunes Store customers.
iTunes Store Discussion Board (Mac)
iTunes Store Discussion Board (Windows)
I have been unable to connect for more than 24 hours on a Mac. If you haven't been able to connect to the iTunes Store for more than a day—and other customers on our discussion boards aren't experiencing similar issues—a software or internet service provider (ISP) configuration issue may be blocking your access to the iTunes Store. Click here to find out which ports and servers need to be allowed for iTunes Store access.

• Your copy of the iTunes application may be out of date. If iTunes is out of date, it will not be able to connect to the iTunes Store. You can download the latest version of iTunes from our website:
  http://www.apple.com/itunes/download/
  
• You may need to reset your keychain. Connection issues are occasionally caused by keychain issues. Click here to find out how to use Keychain First Aid to resolve this issue.
  
• You switched to a new ISP. If you didn't install new networking hardware, your ISP may be blocking access to the iTunes Store. Let your ISP know about article TS1629 which can help your them configure their servers to allow iTunes Store traffic through.
  

I have been unable to connect for more than 24 hours on Windows. If you haven't been able to connect to the iTunes Store for more than a day —and other customers on our discussion boards aren't experiencing similar issues— a software or internet service provider (ISP) configuration issue may be blocking your access to the iTunes Store. Click here to find out which ports and servers need to be allowed for iTunes Store access.

• If you have not done so, try the steps in this article to troubleshoot iTunes Store connectivity in Windows.

• Your copy of the iTunes application may be out of date. If iTunes is out of date, it will not be able to connect to the iTunes Store. You can download the latest version of iTunes from our website:
  http://www.apple.com/itunes/download/
  
• You upgraded to Windows XP Service Pack 2. Windows XP Service Pack 2, by default, enables the Windows Firewall. As with a third-party firewall, you will need to configure Windows Firewall to allow iTunes to access the iTunes Store. To do so:
  
  1. From the Start menu, choose Control Panel.
  2. If the Control Panel is in Classic View, skip to step 3, otherwise open Security Center.
  3. Open Windows Firewall.
  4. Click the Exceptions tab.
  5. Select the iTunes checkbox to enable it as shown here:
  6. 1. 
     2. Click OK.
     If you have further questions about configuring the Windows Firewall, contact Microsoft's Service Pack 2 hotline at (888) SP2-HELP. Read this article for compatibility information.
     
  7. You switched to a new ISP. If you didn't install new networking hardware, your ISP may be blocking access to the iTunes Store. Let your ISP know about this article which can help your them configure their servers to allow iTunes Store traffic through.
     
  8. The "hosts" file may be blocking the iTunes Store Instructions for Windows 2000 and XP: Find your hosts file. The hosts file is normally found in the following locations:
     
     Windows XP: C:WINDOWSSYSTEM32DRIVERSETC
     Windows 2000: C:WINNTSYSTEM32DRIVERSETC
     
     
     1. Select the hosts file. Then click the Edit menu, and choose Copy.
     2. Then, click the Edit menu again, and choose Paste. There should now be a duplicate of the hosts file named "Copy of hosts".
     3. Drag the original hosts file to your desktop.
     4. Right-click the "Copy of hosts" file and choose Open. You may be prompted to choose the program you want to use to open this file. Double-click Notepad.
     5. When Notepad opens, click the Edit menu and choose Select All. Then click the Edit menu, and choose Delete.
     6. Click the File menu, and choose Save. Close Notepad.
     7. Right-click the "Copy of hosts" file, and choose Rename. Type "hosts" (without quotation marks) and press Enter.
     8. Restart your computer and try connecting to the iTunes Store.
  • Make sure that your computer meets the iTunes minimum system requirements.
  • Make sure that your operating system is up to date:
    • For Mac OS X, use Software Update to determine if you need to update your system.
    • For Windows, go to Microsoft's Windows Update webpage.
  • Make sure that you can access the Internet on your computer.
  • You may have firewall or other security software that is preventing your computer from connecting to the iTunes Store. Click here to find out which ports and servers need to be allowed for iTunes Store access.

try to go on apple.com and see if your info is correct. someone might have taken your account over

http://discussions.apple.com/thread.jspa?threadID=1616699

I had added a proxy to my internet explorer while trying to access another intranet. I always use Firefox so I didn't think it would matter if it was set to the intranet IP as the proxy, but apparently itunes and some other programs blindly copy the proxy settings from explorer and will stop working. everything was fixed when I disable the proxy in the connection tab.

just disappeared from itunes or gone completely ?

If just disappeared from itunes you might want to check in my documents --> my music or my videos and see if there any itunes items in there.

If not then go to itunes --> edit --> preferences and check where its storing your itunes data on the advanced tab unless its on mac os in which case go to preferences --> advanced and check to see where its storing the data.

You can then go there and see if your downloads are still available.

If not and you did not back them up then im not sure if there is much you can do - you can try and call apple support and see if they will let you download them again ?

I think the general idea of itunes is that you back your data up frequently to ensure you have it. That way when you re authorise your computer if you ever have to format or re install then you always have your downloads handy to put back into your itunes library.