What happen if laptop motherboard is gone

HELLO
my hp pavilion dv6500 is working, but screen is blank.I tested with another monitor,tested memory sticks and slots,but the same result.Can in be motherboard or graphics problem?

Since generally everything on a laptop shy of memory and hard drive is on the motherboard, then most likely there is your cause.

Now how to fix, unless the laptop is still under warranty your best bet is to replace it completely. Motherboard replacements are quite expensive.
And of course with a newer sandy bridge or ivy bridge laptops you will have much better performance as well.