If the Dell 9020 Mini Tower came with an Intel APU Integrated Graphics I would remove the HD7750 GPU card and reboot and see if the Intel Graphics is working.
By removing the HD7750 GPU card BIOS will automatically make the Intel Integrated Graphics the Main Display Device or Adapter.
So you should have video output from the Motherboard video output ports.
If you do have video from the Motherboard's video outputs then I would delete the current AMD Driver using DDU with the internet disconnected and the AMD Installation folder deleted at C:\AMD.
Reinstall the HD7750 and see if you now again have video output from the GPU Card.