What could my hamsters died of?

Ok so i went to my friends house the other day and she had a little hamster that she didn't even took care of and i felt pity for the little hamster and told her to give it to me that i was going to take care of.. i took it home and putted it on a box and gave it water and food..and putted toilet paper on the cage because supposedly they like to chew and i read online that hamsters like to chew on that....then i went to sleep and in the morning when i waked up my mom told me that it was dead..she told me she had putted it on the sun for a while because she though that hamsters liked that..and then she put it inside the house and at that point it was alive but then like 10 minutes later it was dead..im really sad and feel really bad for the hamster... when i saw him his eyes were open and it was like popping out a bit with something white..and its mouth was open..it was horrible :(..what could he had die of?