well that can be 2 things. one: teleported him self with cell to the kai's or w/e planet so the exsplotion wouldnt damage earth. two: he came back and help gohan out with suport and stuff.
He got defeated by Cell if I'm not mistaken following Cell absorbing Android 17 and 18. Gohan then avenged Goku and killed Cell. If I'm not mistaken. . .
Nah, it was a strain of heart diesease that he caught. The ironic thing about it was it was a "natural" death. He wasn't poisoned or anything, he died like most mortal men.
Goku died by taking cell to king kai planet and sacrificed himself to kill cell. Cell did not die. Goku lived through the heart attack. Gohan avenged his father by killing cell.
I know, he never died from the heart disease. Only in the future did he die from it. Like sora said, cell was about to self destruct and Goku teleported him to King Kai's planet.