Use an install disk to repair the hard drive, verify permissions, repair permissions and such. Also try Disk Warrior which can sometimes repair problems that Apple's disk tools can't. If that doesn't work, reinstall OS X. If you don't have a disk go on eBay and buy them. Just make sure that your model is compatible with the set you get. Alternatively you could call Apple, which I believe sells them for about what eBay does, so that's an easy fix too.
http://support.apple.com/kb/ts1440
If you see a flashing question mark when you start your Mac, it's probably because it can't find the system software it needs to start up. Usually, all you have to do to get your Mac back up and running is remind it where its software is.
If your computer starts up normally after a brief delay, you probably just need to reselect the startup disk in Startup Disk preferences. It's normal to see the flashing question mark when a startup disk has not been selected. In most cases, reselecting the startup disk is all that is required to resolve the issue.
Sometimes, your computer may not start beyond the flashing question mark.
Tip: If your computer has a gray screen (with no flashing question mark) startup issue, see Mac OS X: Gray screen appears during startup.
Check the mouse and keyboard
This issue might occur if a mouse or trackpad button is pressed during startup. Make sure the button isn't pressed.
If the issue persists, shut down your Mac with its power button, disconnect any external mouse and keyboard, then turn on your Mac with its power button. If the flashing question mark issue persists, reconnect the keyboard and mouse.
Additional steps
If your Mac still starts to a flashing question mark, follow the steps below. If any step resolves the issue, you don't need to continue to the next one.
MacBook Air note: On a MacBook Air, there are two options for starting up from Mac OS X media: Either connect a MacBook Air SuperDrive to the MacBook Air via the USB port and restart the computer, holding down the C key during startup, or use Remote Install Mac OS X to startup from a system software DVD that's located on a partner computer. Once started up from Mac OS X media, skip to step 3.
A flashing question mark appears when you start your Mac
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Important: If Disk Utility finds issues it cannot repair, you may need to back up as much of your data as possible (or use Time Machine to back up to a different disk), then erase the disk and reinstall Mac OS X. You should back up important files and data before erasing a drive. Erasing deletes everything on the hard disk (including things on your desktop). Also, you can install Mac OS X onto an external disk, start from the external disk, and use Migration Assistant to transfer items from your usual internal Mac OS X startup disk to the external disk, then erase the internal disk and reinstall Mac OS X.
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Computer stopped and froze while downloading an upgrade. Now, it will not boot up. It stops at the gray screen with the apple logo - nothing more.
when i go to turn on the computer all i get is a gray screen
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