That sounds horrible. However, are you sure they were dead? What did the temp get down to? I ask, because I have seen some balls get so cold that they appear dead, but after warming up a bit, they will start to tongue flick and will make a full recovery. It is possible that they got cold enough to slow down bodily functions to almost a stop. Unless it got down to below 40 degrees for 6 hours or more, I would suspect that they might still be alive. If you haven't disposed of them yet, warm them back up before you make the final ruling on death. Then get that new t-stat.
Good luck, and let us know how it goes.