No on can really be sure what you did, but 1: Read the messages that appear on screen.
2: Don't turn off the console while it's doing important stuff... you're lucky it's just your save file you lost.
1. It was right after selecting my hard drive and you always have to just press A to continue, it just asks the same thing as always. I always read messages but selecting the right hard drive is just pressing 4 or 5 times on the A button. Apparently I pressed it another time and therefore it selected 'Overwrite'. I'm just curious what the message was and what the other option besides 'Overwrite' is.
2. I didn't turn my console off while it was doing anything, I turned it off when I was in the Awesomenauts menu when I saw my level was 1 again and when my stats were 0.
Apparently my save file got corrupt but I don't know why it happened. Didn't take out my hard drive, didn't move my Xbox, just wanted to play on a normal Sunday night.
Edit: I suppose there's nothing to do about it. Starting fresh all over again seems fine, but my nickname is still in the leaderboards with the prestige sign next to it... Oh well, too addicted anyway.