Failed to open project?

A friend of mine has tried to run their game today, but it doesn't seem to be working. First, their game opened with a black screen. After that, they asked for my help and sent me the file. I wasn't able to open it. Construct gave the error message "Failed to open project. Check it is a valid Construct 3 single-file (.c3p) project."

After a bit of troubleshooting, my friend encountered the same issue. They still had their original file open, but that wouldn't load their game layout.

I suspect their file might have corrupted, but according to them it was working just fine an hour ago. I've tried copying the file and opening that as a .zip, and I'm able to see everything just fine there, but it still won't open.

Any ideas?

There may be a hundred different reasons. Press F12 and check messages in browser console, it will tell you exactly what is wrong with the project.

I'm seeing this question asked almost every week here. Developers should really add more info to that error message.