I think, I've just rebased a branch on itself and now it's kinda cycled (writing 'rebasing' for more than half an hour). How can I fix it?
Ctrl+C to kill the git command if you're using a command prompt. If Ctrl+C doesn't help just close the app you used.
After that, go back to your git folder and check where you are and in the 'worst' case just abort the rebase with git-rebase --abort.
You can always also git reset --hard to get back to pre-rebase state.