It wouldn't make sense to climb back up into IMC after a successful recovery from inadvertent IMC, particularly as it shouldn't have been difficult to remain clear of cloud and return to the point of departure. Maybe a technical malfunction with the aircraft resulting in compromised control authority? Even that one is hard to contemplate if there was no radio call to broadcast that fact. Pilot incapacitation is another possibility. Not much left of it to work out what might have gone wrong.