Have replaced my bios battery, know the comuter does not boot at all?

I replaced my bios battery, and my laptop does not boot at all. The power light comes on, sometimes the harddhisk, but the screen remains blank, and the hard disk is not booting.

Have check the hardrive on another machine,its fine. But the screen remains blank, have tried to check if this will come up on an external monitor, that remained balnk. Any help most appreciated