Yesterday, i changed my processor to my older one because i want to try if it is working then suddenly when i return my processor to my orginal one, my computer doesnt work anymore, it suddenly shut itself down after 5 sec.

May be because of some interconnection, try it replaceing motherboard with another one

Before going to the expense of a new motherboard, try clearing the cmos first. The old CPU may have changed some timing settings in your bios.

it shut down in 5 sec ,sounds like heat problem ,make sure the cpu has some thermal past left on it and the heatsinc is connected properly,