Sure
If the guy had a heart attack or something like that, then the plane was basically out of control. Still, I don't know where the fascination some people seem to have with a microphone comes from. I don't know how many times I've read a news report that "the pilot didn't report an emergency" or "no further radio contact was made".
Look, if you're having problems, FLY THE DAMNED AIRPLANE! Sometimes, you're just too damned busy (or distracted) to tell the world about your problems. Declaring a problem or emergency is fine if you have the time but it won't make the problem go away.