I've been in the exact same spot under very similar circumstances. What I can add is that the github support was great, we got it resolved within the working day.
The repo was "deleted" to free up the private repos of my colleague but we didnt know that the fork on my account would go down with it. Its been probably a couple months till we got back to it and noticed the issue. They managed to recover the repo and unblock my fork. I just did a fresh repo and moved everything over.
While it is odd behavior, if its really bad they will probably manage to get you back on your feet.
Also Im not affiliated with github in any way just have some repos from student days over there. If it matters im on gitlab and google repos