computer will not power on.

Hi, I have posted this problem  recently but the situation has changed and I am freaking out. In the past my computer was hard to start. I had two press the power button several times before it would start. The fan would come on briefly then stop then turn briefly a second time.I was advised to unplug the machine unplug the power from the motherboard, take out the battery, plug the power cord back in and see if the fan would turn. It did not. Now when I put everything back together and press the power button the fan will turn as long as I hold in the button but stops as soon as I let go off it. Any help will be greatly appreciated. Thank you for your time.