Do you know the location and/or names of the video clips that have been lost? Try searching for the clips using Windows search function if you cannot remember the location. Note the location, and when you open your project, use the browser that pops up when it asks for the lost clips, and point to that location. From there, it should find and insert the clips.
If you have trouble inserting the clips, but have found them through Windows search, try playing them in media player or alternative to ensure they are not corrupted.
I'm not sure if Premiere makes automatic backups of the project file, so if your project file is corrupted, you may have to start anew.