Can't open iMovie library.

Something that worked for me in this situation is that I right-clicked on the imovie library and clicked on "get info"

At the bottom under "Sharing & Permissions" I had to add myself to read&write (you may have to unlock first to make changes). Anyway, it somehow worked for me.

I had the same problem and found a simple fix.

I searched for my iMovie Library in Spotlight, clicked it, and it automatically opened iMovie with everything working as normal.

Hope that works for others, as well!

I have the same exact problem. In fact, iMovie, iTunes and Photos are all not working after upgrading to Sierra.

Try booting into safe mode by restarting your computer while holding down the shift key. After boot up is complete open disk utility and run first aid on the disk that contains your iMovie library. Then reboot normally. See if this helps.

As it turns out (called in a Mac expert as it seemed like all was failing, including Time Machine), the issue had to do with my external drives (both La Cie, one as the primary drive for Movies, Pictures and Music and the other for Time Machine). Turns out, the raid software on those external drives was apparently not compatible with Sierra and made it appear like iMovie, iTunes and Photos files were corrupted. Right now, my Photos library appears to be repairing (has been 3+ hours as file size is 700 GB) and iMovie libraries appear fine. However, iTunes (nearly 1 TB) is only recognizing half of my songs, seemingly because I have an iTunes file buried within the iTunes file (its not recognizing the first one). Having two iTunes files within each other was something that happened during an iTunes upgrade many years ago. Trying to figure out that one.

Glad your iMovie is now working ok. I am not knowledgeable about iTunes so can't offer any suggestions. Have you posted on the iTunes forum where those experts hang out?

I'm confused. What did you do to finally access your iMovie files?

Thanks!