Android Studio - All file names in project are red but there are no errors

Perhaps you are seeing the File Status colors that Android Studio uses to show the status of the file. Open Settings and go to Editor > Colors & Fonts > File Status. Here you can see what all of the different colors mean. My guess is that your reddish files are that color because you have not yet added them to source control.

Just simply right clicked the file -> GIT -> Add.

I had the same problem: everything was in red and my MainActivity was not found. I resolved it doing:

File -> Invalidate Caches / Restart