How do i undo format

The format you performed erased everything!!!!!!!! Unless you can get a hold of some software to undo what you did, you are SOL

All I can think at the present is that you either have a software/firmware issue or a virus. I would suggest scanning your device with the latest anti-virus (If you haven't already), or downgrading your firmware to an earlier version. Below the link for AVG:
https://play.google.com/store/apps/details?id=com.antivirus

MP3 players can only play music files in mp3 file format. You can download mp3 music files from the Internet toyour computer and then copy them to your MP3 player. For your CD collection, you need to convert your CDs tomp3 music files onto your computer or download them from the Internet and thencopy them to your MP3 player.
You can do this using Windows Media player and Rip the CDs - Media Player willcreate a MP3 music file for each music track.
Click on this link for instruction on converting music files to mp3 file format:- http://www.fixya.com/support/r5798418-creating_transferring_mp3_music_files

You can go into the settings option of the player from the menu, and scroll all the way down and you should see an option of format. Then it should ask you internal or external and select internal and it should delete everything on there back to normal.

In order to get rid of it you will need to format it..

there are basically 2 ways.

there is a format option located somewhere in the menu / settings options of your mp3 player

2, connect it to your computer (assuming your using windows here )
it will show up in mycomputer as a removable disc
right click on it and click format..

WARNING formatting will delete everything on it, please backup up your songs before you format if you dont want to loose them

Try this: disconnect the player from the computer, hold in the volume + button, and reconnect the player to the computer while holding the volume + button in. Keep holding the volume + button for an additional 5-50 seconds. Windows should eventually go into recovery mode and revive the player- it will pop up a window that says Found New Hardware Wizard.
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Download the Sansa Recovery Tool from the Sansa website.
Format - Erases Everything: Connect your Sansa player to the computer and open Windows Media Player. Click on the Sync tab. Pull down the drop down menu underneath the Sync tab by moving your cursor over the Sync tab and left clicking on the little black arrow underneath the Sync tab. Highlight "Sansa 1GB" (wording may vary slightly), and then select "Format." - this should erase everything.
Firmware Upgrade- Erases Everything: Get the latest firmware upgrade from Sansa - it updates the firmware and erases every song in the player all at once. Note: this method doesn't work if already have the latest firmware. Go to this website and click "Firmware Updater" to find out: http://www.sandisk.com/Retail/DriverDownloads.aspx
or
http://www.sandisk.com/DriverDownload/driverList.asp

-Tha Mp3 Doctor

Sounds like you need to reset your sansa, should be in the settings under factory reset, and/or format. THis will delete everything.

The following procedures will erase all songs on the player.
Check if the file system is NTFS. If yes ,format it to FAT or FAT32. - Connect player to computer, click my computer, right click on icon of mp3 player and select format.
You can also format the player in Windows Media Player tiself.
Connect your Mp3 player to the computer and open Windows Media Player. Click on the Sync tab. Pull down the drop down menu underneath the Sync tab by moving your cursor over the Sync tab and left clicking on the little black arrow underneath the Sync tab. Highlight "name of Mp3 player here 1GB" (wording may vary slightly), and then select "Format." - this should erase everything.
You may also want to check
http://support.rcaaudiovideo.com/downloads.aspx?product=25
for any firm updates - the above procedures in some cases undo the original formatting on the player, and a firmware upgrade will be required to replace the Mp3 player's formatting.
-Tha Mp3 Doctor

Go into settings, select mode, then select msc. When in this mode you can look at the player's drive, by going into my computer, then selecting the player. In this mode you can transfer music back and forth from computer to player, in both directions. MTP mode is only used with Microsoft Media Player. If you stick with msc mode, and browsing the unit as a drive, you can't go wrong.

If you didnt delete the music from your trial subscription with Rhapsody, they are still on the device, taking up space. Your device will allow any type of music to be transferred, it doesnt have to be specifically from Rhapsody. Best idea is to "format" the device to remove everything and try transferring the music onto the device. Connect the device and go to My Computer>Device Name, then right click on the device and select format. This will erase everything. If you are not able to format the device, change the mode, then try again. Device>Settings>mode or USB Mode. Reply if you have more problems. Best of Luck.