my hp old p4 computer says fan not working can i salve this ?

I don't know if you speak about the fan that sits on the processor. If you replace this one you have make just the right step ! Bigger fan, cooler processor ! Now to your problem. If your fan stop working and after that no boot, I'm afraid that you have burn out the processor. The good is that a P4 processor in our days cost almost nothing and you can replace it for about $30 together with the original fan. But it may by another problem about not booting... Please be more specific. What you can see after you try to start your PC ? Can you see the bios setup when you tap the key "DEL" on start-up ?