Larry Dighera writes:

Is it possible that BOTH pilots fell asleep?

Absolutely, but it implies that either both pilots were coincidentally very
irresponsible in not getting enough sleep before flying or not keeping one
person awake at all times, or there is a problem with the airline that leaves
pilots with so little rest that they cannot resist falling asleep in flight.

The Federal Aviation Administration is investigating whether two
airline pilots who flew past the airport in Hilo Hawaii by 15
miles last Wednesday were asleep.

Go! Airlines flight 1002 left from Honolulu and was expected to
land in Hilo around 10 a.m., but had to turn around after flying
past the airport. ...

I'd check the history of the pilots in the days prior to the flight to see if
they got appropriate rest, and if they didn't, either the pilots erred in not
sleeping enough, or the airline erred in somehow preventing them from getting
enough rest.

A radar track of the flight provided by the Web site
www.flightaware.com shows the plane remained at 21,000 feet as it
flew past Hilo before returning to the airport.

Starting the descent out of cruise normally requires action by a human pilot;
otherwise the aircraft will continue to fly its route, but without descending.

The Helios Airways incident was similar: after everyone lost consciousness on
the flight, the automation flew the aircraft to its destination and executed a
missed approach and hold, but never left cruise altitude (until the engines
ran out of fuel).