I'm very sad and shocked to say my False Tomato Frog, Horacio, passed away last night

I've absoloutly no idea what the cause was, and had no preceeding expectations this was going to be the case. I had come home from work, misted him as usual, and he was breathing normally, and looking alert. several hours later, I've gone up to check and he is at the front of the tank on his back

I checked the temps, 73.8 in the warm end, 68 at the cool, so no change there, and he wasn't dry, so no dessication. I'm trully stumped what's caused his death, I wondered about impactation, but he was on plantation soil/eco earth, which I was of the opinion is safe. The only other thing that was a recent change is I tried to feed him on a wax moth, but that moved around above him too much the previous night he lept about his tank any time it came near. Could he have died from stress?

I'm concerned it was anything to do with food/water etc as the sources for all my frogs are the same. My whites seem to be doing fine, but then again so did Horacio.

I'm really devastated at his death, especially so soon after I've started to keep frogs.