My computer was shutting down 2 or 3 seconds after pushing the On button. Turning the power off at the back of the computer for 20 seconds sometime eventually got the computer going (might have needed 3 attempts).
I figured something was not re-setting or was setting incorrectly at the last shutdown so tried the following, and all good now
"Before pushing the power ON button on the front of the computer, push the RESET button twice (on the front of the computer), then push the ON button."
Not sure if its a fault in the reset circuit or the reset button, but it works for me