If you are on 3.0.2, I would suggest downloading and installing 3.1.3 - this corrects bugs in 3.0.2 which might be causing the problem.
It is also possible that you simply didn’t give it enough time. However, I don’t want to second guess what you did; I would try updating the software first.
If the 3.1.3 update doesn’t work, you could try using the corrupted file recovery procedure here: Corrupt or Otherwise Broken Audacity Project Recovery