Loading maps, the cycle of death

Been really happy with the 5x+ which has behaved faultlessly since I bought it. Yesterday an uneventful running activity was correctly recorded but sometime afterwards the watch froze on my normal watch screen and stayed like it overnight.
It wouldnt turn off or respond to any button or be recognised when plugging into the PC or power. A 4 button hard reset just put the watch into a perpetual cycle of turning on, loading maps and turning off. Eventually Garmin Express recognised the watch and I downgraded the firmware from the beta to last public release but with the same result.

A lengthy discussion with garmin support via their cool chat feature has led to an RMA to return the device.

There was no event apparent that could have caused this, no update, drop of the unit or anything I can think of so I thought Id post the issue here just in case anyone else suffers the same frustrating situation.