Hello - Your project file is reporting that it cannot find certain assets (clips, etc.) so it's apparent that the location of these clips has changed or they might have accidently deleted them. You might still find them.
1. Check the Recycle Bin to rule that out; if you see any of them, use the Restore function to put them back where they were. (when you re-open the project, they'll be included and the warning will cease.
2. When the asset(s) cannot be found, you are given the option to Search your PC to track them down. Perhaps these clips got moved after you worked on the project? Browse your computer if you know some likely locations.
3. Failing that, make a note of the filenames missing and use the Windows Search function to hunt them down by filename (no matter where they are) and then either a) move them back OR b) use the Browse to tell your project file "these clips live in this location now". Re-save your project with this info and in the future you'll have all of your elements present, regardless of where they are located.
as a best practice, ALWAYS export your project as a zip [file menu] at the end of each active work session; put that zip archive in a safe directory; when you want to work on the project again, import the zip and proceed; zipping will gather and keep all assets used in the project together
I've always found this workflow to be bullet-proof