RAMPS stopped working from mains. (no replies)

Hi,

Yesterday I have turned on my printer fom its AC switch and saw that RAMPS wasn't working. If I plug the USB cable, everything is operational but without it there is no power going to Arduino. I normally didn't need to use the USB to power the unit on (to use with SD card). What could be the reason for this behaviour?

(Note: I'll check to see if I have unplugged a cable, which could be a possible reason. What I want to know is if there are any other reasons related to a hardware failure which could create this symptom).

Thanks...