Undo a git merge request

I’ve uploaded a file to my fork - Gitlab was so kind to prompt me with a dialog giving me the option to start a merge request. I have not read through or paid much attention to it, but realized what happened after clicking the button.

Now my changes to my private repo’s fork (including the entire commit history) has been sent upstream! I fully understand its due to my fault - but is there a way to reverse this??