Are you sure there's nothing you installed since they were both working together?
Have you done this:
Go to this location:
Find the file titled "com.apple.iApps.plist" and drag it onto the desktop.
Now try to start iMovie. If it works you can drag the com.apple.iApps.plist to the trash.
Otherwise re-installing should work!
P.S. Please try and keep your posts about the exact same issue in the same thread. It's easier for others to find later, and easier for others to keep track of what suggestions have been given and taken and what worked and didn't work.