I don't know if this will show up as an OP blue comment since I've never done this, but yes. I'm the OP haha. So yeah I finished the paper before it was due but I didn't turn it in because I wanted to read it over again the next day just in case. Little did I know I would forget about it completely. (I was busy studying for a bio test and quiz all weekend) I finally realized I didn't turn it in about 4 days after it was due online. I talked to my prof. and he said it should be okay. Keyword "should." He hasn't graded it yet so I don't know if he took points off. Anyways, yes I deserved it haha.

Just so this doesnt happen again, write everything in googledocs. Not only does it auto-save like crazy it has a handy editing history so you could send the essay with that link and you have absolute proof that you'd finished it early. Some will take off much less when they see an honest mistake.

My sister had a similar situation. She finished early and all but submitted the file for another class by mistake. Took her few days to realize it. She then emailed the Prof and Prof let her send in the assignment. He checked the metadata of the file for edit date before accepting. Your prof could have used similar method to make sure you were not lying. Thankfully she did well in that assignment :)