As you can se from the name of the missing file it is not the project file - but one or more of the clips etc in your project,
As musicvideo suggests it is probably some media you have deleted yourself.
In order to get to the lightning bolt icon just open the project file and when the missing file message turns up then select "Ignore missing file and leave it offline" - and you will see what more media might be missing.
Then do as musicvid suggests (click the icon) and then save the project file under a new name - whick, btw. you should do all the time in order to be able to go back to an earlier version.

Thanks so much!!!

Thanks so much I needed the detail you provided. I did indeed find it and resaved it ,issing whatever file was missing, and saved it as the soundtrack so hopefully I wont lose it again.
I didnt find whatever file had gone missing, but it looks ok. I hadnt started actually doing the video bit as I have yet to figure out how I cut the clips? I tend to edit rhythmicly so once teh soundtrack is done I can start in.
There is a Trimming window, but the scissor symbo remains unhighlighted so I can use that to trim. And my cuts can be mere seconds so I really need to see exactly where I am in tne with the music etc. So do you know how I can actually start making the cuts please?
And given what happened here with the missing media is there some special way that one should throw stuff away? Or do you just never delete anything for ear of losing teh project?
And lastly, sorry to bug you, if I have another cassette of video content which I want added within this project, if I were to save that onto an external drive versus the computer itself, then will that be an issue, or should I just try and upload it directly into the preexisting project?
Thanking you eternally!!!! You saved me!!!!

You may do better doing your fine trimming on the timeline. You can use the trimmer to narrow down the section you want to use. Press i at the beginning of that section and o at the end (for in and out), then drag that piece up to the main timeline. Make sure you've included a little more than you'll need.

On the timeline you can use the s key to split, though possibly even better for you is to simply drag the edge of the event in to trim. You can also drag back out to restore what you've trimmed off. Since this is non-destructive editing you can keep playing with the event until you've got it just right, or even go back later and change your mind.

For deleting unused items from the project, you can do exactly what you've done and then remember the lightning bolt icon (clean project media) to clean up afterwards. You can also delete from the project media window, which takes care of the housekeeping for you.

Note that there is no particular relationship between the Project, and all the files that make it up. You can store all the audio & video clips anywhere you want to or where you have room for them. Putting clips "into the project" doesn't mean that they have to be in the same place as the other clips that are in the project. It merely means that they're available to be used on the timeline, no matter where they are. Personally though, i prefer to keep everything associated with a project all in its own folder dedicated to that project. It makes it a lot easier to keep track of everything later on.