Unity character dissapearing when I hit play

So I had a normal game, everything set up the script is set up exactly how it should be, and everything was fine. I tested it beforehand and it was doing great, but then all the sudden a few days later as I get back to work on developing, I pressed play and the character dissapeared. I looked on the inspector on the right whilst pressing play and noticed my y axis was going into negatives. I stopped playing and it went back to it's normal state, how do I stop my character from well.. falling down?