Start by listing your full specs, including your power supply make and model if known.
Does the computer seem to be starting up? Do you hear fans spinning up? If you were switching from onboard to a discrete card, did you switch the monitor cable to the card?
Go back to your motherboard graphics and get into BIOS, see if there is a way to disable onboard graphics and/or turn on graphics to the PCI-e slot. IF so do that and then try your card out.