You may have transferred your iMovie library without first clicking on the Library name in the My Media side bar and doing a File/Consolidate Library Media so that all of the media used in your projects is carried over with the Library. You should also first click on your Project icon in the Project Browser view and do a File/Consolidate Project Media. If you had not done those things, move your library back to your old Mac, plug in any external drives that you were using to store media, and do the above procedures. See if that restores everything.
Also, in the event that your iMovie library got corrupted, you could go to your Time Machine Backup and restore your iMovie library to an earlier point in time and see if that cures it.
Another possible cause for this problem is that your flash drive may have been formatted FAT 32 or EX FAT. iMovie libraries must be stored on drives that are formatted Mac OS Extended (Journaled) or APFS. Otherwise data loss can occur. If you need to reformat you must first back up all data on the drive because reformatting will erase the drive.
-- Rich
