iMovie won't open..

How far did it get? Did the Dock icon bounce? Did it give an error?

Maybe repair permissions from Disk Utility (in the Utilities folder) and perhaps remove the file named com.apple.iMovie.plist from the folder you find it in. Reinstalling iMovie wouldn't have affected this file so if it's corrupt, reinstalling wouldn't have helped.

The iMovie preference file holds your settings for iMovie. If you want to keep them, then keep that file so that if removing it does no good, you can put it back where you found it, replacing the new default version, and keep your old settings (once we get you back into iMovie in the first place).