Not knowing where you are etc in relation to your priest and whether you went to confession face to face I would offer the following:
If you did go face to face ask him the next time you see him what your penance should be/was
Do the penance that he gave you the last time you went to him (if you’ve gone to him before).
In my experience, I’ve never been given a penance as much as a rosary. If you’ve never had a penance that long, then if you offer up something like a rosary for your penance. My logic here is that that should cover your penance, unless your penance was something like go visit the sick or something.
In any case, I would recommend that the next time you see this priest or go to confession to him you bring it up.