Sorry Barb, you've got to toss it. I don't know what's more dangerous in there, the proteins or the cooked starch.
Once something is cooked it has to be held at 140 degrees. It can be held there for quite some time, although I don't know who'd want to eat anything held for all that time...
However, once your casserole was left on the counter, the temperature began to drop below 140. At that point, it could only safely stay below 140 for 2 hours. Once you hit the two hour mark it would either have to be reaheated to 165 or quickly chilled.
You mention you had quite a bit left...(that really is a shame, it sounds expensive). In the future, when you have a lot of something thick and heavy like that, you've got to break it down into smaller pieces in order to cool it quickly enough. Cooling quickly is just as important as holding at the proper temperature.
My kids have left stuff out before. I am sure you'd agree it is better to toss it than risk getting sick for the price of cheese. Did you ever go into the kitchen in the morning to discover someone left the refrigerator door open?