Its recommended to keep your computer plugged in, but OFF, and cooled when you work on it. And the PSU switch OFF as well. This way the computer stays grounded, anbd you a re safe just so long as you never mess witht he Power Supply! Power supply can hold thousands of volts even when not plugged in.
To check temperatures, you turn it on, let it run for several minutes, then probe it. If you want to try with and without fans, you turn it off, let it cool, and unplug the fans, start it up, then probe it again. You must never have the computer running, even when doing simple changes.
Anyway, did you happen to unplug the +5V switch for the power button?