It would help to have a bit more information.

What does the indicator light on your monitor do when you boot the computer? turn on and then stays on....makes a pop and starts blinking?...or just continues to blink with no change what-so-ever.

Did you replace the old card? or did you replace the on-board (built in ) video with a video card?

Wour motherboard or computer model? This is so we can look at the board to see if you have what kind of video slots...PCIe.....PCI.....AGP.....

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As a guess...have you removed the new card and gone back to the old one to see if it works that way?

The other option is to remove the new card completely and then reseat it in the socket firmly making sure it is evenly seated...especially after putting in the holding screw or lock down (if tool-less)....Take care as some cards are a bit long and get easily jostled when replacing components