in KRC4 compact, that PSU is part of common PSU shown as "3"
but... this powers everything so when 27VDC is not present, there is no power for the motherboard or CCU or for smartPad and you could not see those messages. in other words your PSU is most likely fine. if your robot is unmastered, you need to master it.
when you drive a car and run over some nail, the car tire is likely to get flat. but it too was just fine yesterday. 
everything can go bad and usually it does if you give it enough of time - specially if things are not taken care of or when installed or messed with by unqualified personal.
mastering loss can happen when SLA batteries are too low. then you just need to loose power and robot is unmastered.
even if batteries are ok, things can go wrong. one of the common issues is collision.
another one is loose data cable (X21-X31) for example when O-ring is missing, which makes connectors fit loosely and any movement or pull on the cable will cause a problem. This would indicate sloppy installation and result was simply ticking time bomb.
