Do you mean turn on then immediately turn off?
Well, there are a few possibilities. One would be the power adapter isn't working so the computer is trying to start on battery power. 2, the system is overheating. 3, the motherboard is damaged or failing.
How old is the laptop?
however, for just a little bit more you could get a whole replacement: http://www.ebay.com/itm/Gateway-NE56R41u-Windows-8-HDD-448GB-Intel-Pentium-2-20GHz-4GB-RAM-/111790895207?hash=item1a0741b067:g:-bMAAOSwnOJWFEJ0