You're not stupid. It slipped your mind and you forgot. Probably because you've stuff going on. It's OK and normal to forget important stuff sometimes.

Can you still hand it in late? When I did night classes, I'd set alarms before and during the day it was due because otherwise I would forget. I labelled the alarms on my phone too, because I would forget the purpose of the alarm. And I only had one or two assignments at any one time.

What my younger brother and friends tell me about university and grad school sounds like a nightmare with my task management skills.