Custom-built; System turned off but stayed "on"

[Get_off_my_plane]

I've never had this happen before, but I went to shutdown my desktop and everything went fine, monitor turned off, but the computer stayed on. Lights and fans on, but unresponsive. Ctrl+alt+del did nothing. It was this way for over an hour so it's not conceivable that any updates were being installed.

Any idea what could have caused this? I've never had any problems with my build, it's Win 8.1. Intel 4570, rosewill capstone 450w...

 

[alexoiu]

Is there a powered USB device connected to the motherboard? If not, it might be a power supply issue.