Does the BIOS list the card when you boot the system? (Might need to change/disable the BIOS setting from "Full Screen LOGO" or similar to see POST messages e.g. to see the memory check count, disks found, peripherals enumerated etc).

Worth mentioning since it seems possible the card is not being recognised at all by the motherboard hardware...
Ignore the following if you are using a modern motherboard...

How old is the motherboard? Make and Model? Type of PCIe slots...

1. On some very old motherboards - the x16 slot will only support graphics cards
2. Suspect your card is PCIe V2.0. Version 2.0 cards *should* work in Version 1.x slots - but there are a few exceptions
3. Are the slots sharing bandwidth? Two of my motherboards, for example, have this restriction

The PCIEX1_2 and PCIEX1_3 slots share bandwidth with the PCIX4_1 slot. When the PCIEX4_1 slot is populated with x4 card or greater, the PCIEX1_2 and PCIEX1_3 slots become unavailable.