What are your PC specs first and foremost?
Try the following, first, make sure your PSU has the correct watts AND Amps to push your cards safely for optimal operation. Then you can try taking out the CMOS battery from the motherboard as I usually recommend resetting the BIOS whenever you do a major hardware change such as a CPU, RAM, GPUs and such. Third, it could be that your current display (monitor??TV??) could be going bad now. Try connecting your PC to another display/HDTV via HDMI and such. Also, if your current display has a removable cord (VGA/DVI/HDMI or so), then try using another cord to connect your display to your new card. Sometimes, it's a bad cord that give the impression that your GPU and/or display went bad.