no POST

I get no video or POST on a Compaq Presario 5000. No beeps. It just appears dead. Power to CPU and fan appear good (fan comes on)and keyboard lights flash on as do lights on CD-ROM drives. I suspected CMOS battery, but new one didn’t help. Also tried a new CPU. RAM is good, as are all PCI cards, so don’t ask. Blank screen only. Does not go through POST, so I can’t boot from floppy or CD or HDD or get to BIOS. Can anyone help?