iMovie should automatically be saving your changes to the project in the Projects browser (where your projects are displayed as icons). Are you saying that when you click on the project icon in the browser your last 6 hours of work is not saved? Is there anything at all that is saved in the project? Is this issue happening with all projects or just this one?
To cover the bases, make sure that you are opening iMovie in the correct library. Close iMovie and then look in your Movies folder on your Mac (Ho/Home/Movies). Double click on the icon of the library that contains your project. iMovie will open. Go to your projects browser view and see if your project contains the changes.
In case a corrupt preference is causing the problem, try deleting preferences. Open iMovie while holding down the Option
and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
If no success, look in the iMovie Backups folder to see if you can recover your work. 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. Navigate to your projects browser (where your projects are displayed as icons) and see if your project is intact with all of your edits saved.
-- Rich