Hi Hulky,
These are couple of steps I would suggest, which might help us in fixing the issue.
1. Releasing the flea power of the computer.
-- Disconnect all the external peripherals from the computer along with the power cable, monitor, keyboard and mouse.
-- Press and hold the power button for 10 to 15 seconds.
-- Connect only the power cable, monitor, keyboard and mouse and try to turn on the computer.
-- Check the functionality of the computer.
-- If it works fine, shut down the computer and turn it on by pressing the power button and try the process a couple of times.
In case it doesn't work, follow step 2.
2. Bypass the power button.( By pass the Power button if you are comfortable in doing)
-- Shut down the computer, disconnect the power cable from the rear end of the computer and open the chassis.
-- Disconnect the CMOS battery and the cable from the power button to the motherboard.
-- Connect the power cable at the rear end of the computer. Turn on the main power from the wall socket.
-- Check if the fan starts spinning.
** NOTE: Please ensure that the power cable of the computer is disconnected before opening the chassis. Refer to the Service manual for assistance on removing or replacing the internal components of the computer.
Please try the steps and write back with results, would be glad to assist you further.