About a month ago I was cleaning some melted plastic from my hot end while it was printing and poof. I saw a spark and the temperature plummeted to zero. At first I thought that I smoked the RAMPS board but I could have burned up the thermistor. Since I was down for the count I went and ordered both 10 new thermistors and a new RAMPS board on ebay.
I finally had a chance this weekend to sit down and check things out. I used my multimeter to do some testing and it turns out that the thermistor was reading as wide open. I spent a few hours taking the hot end apart and replaced the thermistor and we are back up and running. I guess that I have a few spare parts in the event that this happens again.