Delete Preferences File
It will eventually happen. Something will go wrong and Illustrator will no longer start up, or a tool will act weird and not let you change it’s settings or a modifier key will stop working… There are a number of ways that this issue can manifest itself. Luckily it’s easy to fix.
Quit Illustrator and as you are restarting it:
On the mac – hold option-command-shift or on the PC – hold alt-control-shift
Some versions will bring up a dialog asking if you want to delete your prefs, some versions just delete them.
If this fails to solve your problem you can manually remove the preference folder (yes, folder). It will be recreated the next time you start Illustrator.
On the Mac, it’s located at HD\Users\your_username\Library\Preferences\Adobe Illustrator CS# Settings\(localization folder)\
On the PC, C:\Documents and Settings\your_username\Application Data\Adobe\Adobe Illustrator CS# Settings\ Or something close to that, based on your version of Windows.
You can throw away the entire folder there, or rename it if you prefer, and Illustrator will create a new one in place of the old one the next time it’s started up. Renaming it gives you the option of trial and error’ing the old files back in place if there are certain preferences you’d like to keep.
If all this fails, you can bring out the big guns and try the Cleaner Tool, provided by Adobe. This will require you to uninstall everything from the creative suite and will require a re-install, so make sure you have some spare time (and your install discs) before you go that route. It will remove all the cache files and additional noise installed by the apps to be sure that none of them have gotten corrupt.
If after trying all of these options it’s still failing, let me know below in the comments (I really do read them) or try the links to the right. And if it does work for you, let me know and don’t forget to check out the rest of the site.