The roll out safety switch has gone bad. This is a wax switch that melts and opens when flame comes out of the box and up the front of the cabinet. What causes this is a sooted heat exchanger, being adjacent to a tall wall (As explained in your book), broken firewall tile. The switch itself is a short black piece in the wire running across the bottom front of the heater. The new one will have a loose black covering and when it gets hot it will shrink and get tight. It will just plug into the wire and is very inexpensive. It's job is to protect the rest of the components in the cabinet.
Depending on the problem it can get rather in depth and is usually something I would recommend you get a professional to look at. Some of it can be complicated some of it is a fire hazard. You can try a new roll out switch first to see if this was a fluke. So you could fix it for $20.00 or you could be throwing $20.00 away. Quite often it's worth a try. Good luck.