I put my PC to sleep mode, when I came back, the power led is on, monitor detects nothing. I pressed reset and power botton few times, nothing happens, so I long pressd the power botton, and it cannnot be turned on any more. No onboard LED, no fans.
There is history when I switch my RAM slots, the PC wouldn't turn on, but after I completely disconnected the power for 20 minutes, it could be turned on. Sleep mode also involves RAM, but I am not sure if it's related this time.