Where have you plugged in the monitor?
Previously the monitor would have been attached to the video outputs on your Nvidia card.
Now that you have replaced this with an AMD card, the monitor should be attached to the new card.
If the card is working the video outputs from the motherboard will be disabled.
The card should work at least to the Windows even if there is no driver installed.
If the video outputs on the card are not working, try the outputs on the motherboard to at least get into BIOS and see why the card might not be working.