Holding down the power button on a PC while disconnected from power

Solution 1:

It drains all residual power from the motherboard and memory modules, what that actually resets depends on the design of the motherboard.

As Gateway explains it:

The process drains stored electrical energy in some critical circuits on the motherboard. These circuits start the computer properly.

Solution 2:

I'm not sure if its universal but its also true of thinkpads, and some other systems (according to the comments and this post about HPs ) and apparently even tablets.

Reasons range from ACPI resets, to static removal, to MAGIC. Its worth trying out, but dosen't always work in my experience