Seeking possible answers for surprise and sad death

[Fawkes'Family]

Fawkes was a year and a half old. He has always been pretty small for his age (about 1/2 the size of a typical corn) despite regular feedings. We got him from a private breeder and were very happy with him.

He was well cared for. Had a heating mat on half his tank with a thermometer to make sure it was at the right temp. He had a log/cave to assist with shedding and water bowl in one half his tank. He was handled regularly and almost acted like he wanted to be around people.

This afternoon/evening we noticed he was pretty listless so I put him back in his cage to avoid stressing him out. Two hours later, he was alive but barely moving. We had tried completely cleaning his tank and he regularly has fresh water. However, nothing seemed to work. Shortly after, he passed away.

We are just really sad and looking for some sort of explanation as to what could have happened.