Yup, I'm also using a medium sized (8 x 11) heating pad 24/7. Have it hooked up to a sliding dimmer that I got at Wal-Mart (for all your herp needs!). Got it set so the floor hits 91.8. Has been running for a little over a week now with no probs. I couldn't find a human heating pad no matter where I looked that didn't turn itself off after two hours (safety reasons) so I wound up paying about 20 bucks for the one I'm using from a pet store. Maybe yours cooked from running full tilt the whole time.........or do you use a dimmer?
Your problem has got me thinking that I should go and get a spare in case the one I'm using craps out........
How are you checking your floor temp over the heat pad? I use a simple digital oral thermometer. Only cost a few bucks at the pharmacy and has been quite reliable.........just stick the sensor end under an edge of the hide to hold it against the floor nice and firmly for a few minutes, then take a reading.
However, I am saving my nickels for a handy dandy temp gun!