I'm not quite envisioning your situation. Can you provide more details. When you say "files" do you mean the projects themselves or the media that is contained in the project? What exactly do you see when you open iMovie? And what is the exact wording of the message you are getting? Are your project icons displayed in the project browser? When you open a project, do you see clips with a ? mark in them and a "missing file" designation? It would be helpful if you would post a screen shot of what you are seeing.
To cover the bases, if your iMovie library is located on an external drive make sure the drive is formatted properly and is plugged into your Mac.
In iMovie 10, the projects and the media are located in the iMovie library. Usually the library is located in the Movies folder on your Mac, but it can be located anywhere, such as in an external drive. If the library is located on an external drive, the drive must be formatted Mac OS Extended (Journaled) or APFS. You can see the projects when you control-click on the library's icon in the Movie folder (Go/Home/Movies) or on the external drive and select Show Package Contents from the drop down menu. The projects will be displayed in folders, but you cannt play the projects there. The media is contained in the Original Media folder that is contained in each Project folder. However, do not move or modify anything in the iMovie library Show Package Contents or you risk corrupting the library. Access the library through the iMovie application itself.
-- Rich