Attempt ot start up in Safe Mode, by Holding the Shift key at Satrtup. This takes several minutes while it does a disk check, then demands your user_id and password, even if you normally auto-login.
Safe Mode does things differently, with two impacts:
1) It loads only a minimal set of extensions, including NO non-Apple extensions. If the computer has been mucked up by third-party extensions, it will begin to behave itself in Safe Mode. Safe Mode can be used to change settings, and Install and Un-Install software.
2) It bypasses "automatic" screen drivers, continuing to use the built-in methods of screen-drawing. If you have a graphics Hardware problem, you will see screens in Safe mode that were not showing in "regular" mode.Mac Pro (Early 2009), Mac OS X (10.6.8), & Server, PPC, & AppleTalk Printers
Currently Being ModeratedDec 6, 2012 10:14 AM (in response to Grant Bennet-Alder)
If Safe Mode does not restore joy, may need to do drive maintenance and see if that helps.
Restart and hold down Command R prior to the chime. May take even longer, if it does Internet-based Recovery Mode startup. Eventually a languages select page will pop up, choose as desired. In the next page, choose disk repair, the bottom option. There, click on the xxx.x GB APPLE HDD xxx item on top of the left column to select, then click on Repair Disk. Now click on the Macintosh HD volume below it and do Repair Disk again. Redo if errors appear. Finally, click on Repair Disk Permissions. No need to redo if this last item displays errors.
Quit (twice) and let the Mac attempt to start up normally. Hopefully all will work then. Else, holler back.
FYI, that was OS X Recovery Mode. More info: http://www.apple.com/osx/recovery/