iMovie library lost after upgrade

Yesterday I installed the new version of IMovie. When starting up the first time iMovie needs to to upgrade your existing library. After a while the library upgrade got stuck on 1 project, but I paciently waited all night for iMovie to finish the upgrade. This morning, when there was still no progress, I forced quit iMovie. Now I have an empty library and would like to know how I can start the library upgrade process again. I store all my data on a time capsule drive, so I can get old files back from backup.