Hello,
I assume you run Lightworks V14.5 on Windows.
Lightworks uses an internal media space to store link files to linked media, and copied and transcoded files. It needs at least one valid media location. On Windows this is by default:
C:\Users\Public\Documents\Lightworks\Media
To fix your problem, please try: In your project, left click on "Project" upper left hand side, open the panel "Media.Locations", click on "Add", navigate to the path mentioned, and click "Ok".
The question is, why has the default location been lost?
Could you zip and post the 'Logs' folder, please, it might help finding the casue?
C:\Users\Public\Documents\Lightworks\Logs