My hamster died yesterday and I can't seem to figure out why.?

[Karli]

When I went to visit with him, I noticed that his eye was closed and his nose sounded a little stuffy. I didn't notice this until later in the evening so I couldn't take him to a vet to be checked out just yet. I cleaned his eye with a warm wash cloth and I gave him some cod liver oil. He seemed to be okay, he was eating and still running around pretty fast. At about 5 am my fiance said that he cleaned his eye as well and that he seemed to be doing okay. By 8 am, he had passed away. Does anyone have any idea of what could have caused this to happen so quickly? He had been sick once before but recovered fairly quickly, we kept him in a small room that had constant temperatures and his cage was always clean. We took very good care of him so we are pretty shocked that we lost him. Needless to say i'm heartbroken and some answers would be good. Thanks!