A dead battery could do that. So check you got it in right side up and if that fails put the old battery in.
I replaced the CR2032 battery on the motherboard, without difficulty. I should have downloaded the instructions on doing this from the excellent HP/COMPAQ website. What I should have done next was press the START button BEFORE switching on mains power. When mains power is switched on, the computer switches itself on, but does not BOOT-up. The VDU is blank and the HDD activity LED is unlit. How do I recover the system.