I wonder if there is a cracked PCB track, and cooling it down a lot, by leaving it off for a long long time, will lower the temperature and cause things to contract and could cause a crack to close - highly speculative. If/when it fails again, try blasting its innards with freeze-it spray - that would be an interesting experiment. But anyway, sounds knackered in that case.
Actually, if you flip the board over, can you see any bad soldering? assuming there are some things that are non-surface-mount.