Hi,
Not to worry. There are some things we can try to solve this problem.
I suspect that your iMovie was updated along the way and has opened in a new blank library with nothing in it. Close iMovie and then go to your Movies folder on your Mac (Go/Home/Movies) and find the icon for your old library. Double click on it and iMovie should open in that library with all your projects intact.
If your library were stored on an external drive, make sure that the external drive is plugged into your Mac.
If you don't see your old iMovie library in the Movies folder, then open iMovie while holding down the Option key. A dialogue box will appear listing the available libraries. Click on one to open in that library. Or, if you know where your library may be located, navigate to its location and open it from there.
In the event that the above doesn't solve the issue, try looking in the iMovie backups folder. To
get to the iMovie Backups folder, follow this file path from the Finder menu:
Go/Home/Library/Containers/com.apple.iMovieApp/Data/Library/Caches/iMovieBackups
When you get there you will see a list of previous backups of your iMovie library. Click on one dated just before your issue started. iMovie will open in that library. When iMovie opens, navigate to your projects browser (where your projects are displayed as icons) and see if your projects are intact. If the first backup doesn't do it, then open other backups until you find one that opens iMovie with your projects.
-- Rich