I think I found a temporary solution but it's a bit tricky.
If your Bitsy does what mine has done and you want to work on your game, open an incognito window in your browser and run Bitsy in there. It will load the default game and now you can load your own game and work on it as normal.
Here is the tricky part: because it's an incognito window, if you close it, it will erase your game. Make sure that you save often :)
I'm not sure what borked my Bitsy. There must have been something wrong with my game itself. When I ran Bitsy in the incognito window, I copied and pasted fragments of my game data from the borked file (like: first the colors, then the rooms, tiles, and so on) and it turned out there was something wrong with my rooms. As I look at the code, they seem fine but whenever I copied them to the engine, everthing went nuts. I'm not sure but I think I remember copying and pasting code from one room to another before it all happened, so maybe that was my mistake :P