Go into the folder where the repository is (is that the project folder, I don't use projects) and run git fsck
that will check the integrity of the folder contents.
You may get some 'dangling blob' messages but they can be ignored.
Of course you don't need to re-install ubuntu. The problem is only related to the specific project (unless you have a hardware problem that has corrupted the repository).
Have you got the repository linked to a master on github (or other)? If so then you can re-clone it from there, after making a backup of your flows file.
Otherwise you can re-construct the project from the flow files and others in the project folder. I will have to leave it to others to say exactly how to do that as I don't know exactly how projects work.