I got to looking around with a flashlight and found that screw; but the nut wasn't on it! I started looking around the floorboard and found it under the seat.
It wasn't cracked thank goodness! I popped it back on there. I bet when I frammed on the footrest it rebounded and knocked it off. The pop sound it made when I put it back on there was the same sound I heard that day!
Thanks for pointing me in the right direction; I can already tell this place is going to help me a ton!