The fact that you aren't seeing any topics in the Project Manager pod indicates something has gone wrong.
Do you have a working backup you can revert to? This would be the easiest fix.
Is the project on you C drive or on a network/cloud drive? Classic projects really don't like being on a network drive. This won't fix your current project, but in future, make sure it's on your C drive, and preferably a short path, like C:/My_Projects.
If you don't have a working backup, it could take some work to repair the project (then we can look at the compile error once that's done.)
This tip is unlikely to work, but you could try it. Just create a new blank folder in the Project Manager pod. Sometimes this can force RH to regenerate it's database and cause missing files to reappear. If the project is large (we're talking a thousand or more files, not a couple of hundred), it might take an hour to regenerate - just wait for the thinking icon to vanish. Once it finishes thinking, either the files will have reappeared, or the trick hasn't worked. In either case you can delete the temporary folder.
If the tip doesn't work and you don't have a working backup, let us know and we can walk through next steps for trying to get your project working.