Computer powers on but then turns off after a few seconds

I know this is a late response but someone might have this similar issue which is generally caused by a faulty power switch on the case of the computer. The switch is stuck in the pressed or active position so when you power the computer the switch still being depressed or faulty turns the computer right back off again and you're stuck in this loop. The best way to identify if this is the issue is to remove the power shunt from the jumper then use a screw driver to jump the power connections that would turn the computer on... if it stays on the switch or entire case needs to be replaced whichever is easier

I know this is a a very late relpy but the suggetion above sloved my problem. As TXpaulsr mentioned, unplug the power cables from the jumper then use a screw driver (with ground) and power on the computer. If it fails then you have a faulty motherboad or power supply

When i turns on my computer and leave it few sec and it automatically turns off

Change the power pack with another power pack if you did that and he did not work check the jumper setting if it is okay after that u can now check the ram