I rebuilt my gf's old Acer Aspire today with a new motherboard (ASRock 880GMH), processor, HD and RAM. I kept the power supply, old hard disk and DVD drive hardware. Windows 7 installed onto the new drive without any problems, and after installing the LAN, audio and other drivers (but not the full driver package) from the motherboard CD, it seemed to work fine, for a few hours or so (I installed Firefox and Dropbox and Avira).
However, we went shopping for a bit and when I returned the power LED was flashing. That seems reasonable for a sleep state, so I tried to wake it up using keyboard, mouse and power button (the box has no other buttons); it's not waking up. Even weirder: even if I disconnect the power supply from the wall socket and reconnect it after 30s or so, the power LED resumes flashing (but the box still won't boot).
How can I reboot or wake it up? I guess I could pull the battery from the motherboard and try to prevent it from ever sleeping again, but I'd prefer to understand why it's not working the way it should be.