It's worth a shot.
edit: come to think of it, I had a very similar problem on my laptop. It was in sleep mode, tried to turn it back on, and nothing happened. OS wouldn't load or anything. As I recall, I ended up having to boot from a windows install disc before I could finally restore the laptop to factory settings. Unfortunately, my bios was the only thing working during that incident.
I'm unable to access any BIOS or anything, so is loading from a Windows Install Disk even possible? This machine's mobo has its own Asus Operating system and start-up screen, and even that doesnt come up.
Power the PC and Monitor down, then unplug them both.
Wait about an hour and plug them back in and give it another shot.
One of the first things I tired, no change.

So...did she turn it off at the socket on purpose? Did she think "Sleep + Unplug" was a better alternative?
She's not an experienced PC user by any possible definition. She just didnt think.
Well I can now confirm every time the computer is turned on, the system turns on, the fans come on, the lights come on, then everything immediatly turns off 10 seconds later, and then the whole thing reboots (but with nothing actually, you know, booting). Would I be right in thinking that this is possibly a mobo or psu failure?