Easiest way is simply to re-import the clip from your original source location.
To verify whether the clip is in the iMovie library, go to your Movies folder, or wherever you have your iMovie library stored, and control-click on the library's icon. Choose Show Package Contents from the pop up menu. You will see a list of your projects. You cannot open them there, but the project's media is contained in the project folder. . Click on the folder that bears your project's name and then click on the Original Media subfolder. Is your missing clip shown there. If so, do an Edit/Copy of it and then Edit/Paste it onto your desktop. From there drag it into iMovie. NOTE: Do not move or alter in any way the contents of the Show Package Contents folder. You can copy, but not move.
If still no luck, try opening a previous backup of your iMovie library, that is contained 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
NOTE: If running Ventura or later, in the above file path substitute iMovie for com.apple.iMovieApp. So, for Ventura, the file path would read:
Go/Home/Library/Containers/iMovie/Data/Library/Caches/iMovieBackups
When you get there you will see a list ofprevious 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 projects are intact and your clip is there.
-- Rich