My husband is 86 and was unable to wake up.

My husband is 86 and sleeps 12 hours at night but this morning he got up at his usual time, 8 am, and said he didn't feel well and wanted to go back to bed. Usually, he will get up after a couple of hours but this time, I couldn't rouse him at all. I talked to him, shook his shoulder trying to respond. He was breathing, warm to the touch but just didn't response. After 6 hours, my son came over and between the both of us, got him up where he ate a little breakfast. Very groggy and when I called his doctor, the nurse said call an ambulance if he didn't respond. He has no medical problems, no diabetes, no heart problems, no high blood pressure. There has to be a medical reason since he has only done this once. Anyone have an idea why this will happen?