Just reverse what you've done.
- Put your old card back in,
- download and install the latest version of DDU (download links are here: Troubleshooting Common AMD Graphics Driver Installation Problems ) and use the "Safe Mode" option. Select the complete removal and shutdown option. Make sure you do NOT click on the "Install Default drivers" option in DDU.
- With the system off at this point, swap out your new video card and boot.
- Download and install latest (FULL version) drivers directly from the AMD website
If you still have that issue, check cables etc. This is all I can offer at this point without more information otherwise consider adding INFORMATION REQUIRED WHEN POSTING A QUESTION
PS: Move this to the appropriate Graphics forum; this is the "General" forum.