First, make sure you've plugged the monitors video cable into a video port on the graphics card. Second, need more details - what exactly did you change, what make/model of GPU, what model monitor, what connection are you using on the monitor, etc?
Hey sorry didn't mean to downvote you firstly, on my phone and tapped accidentally haha I'm using a HDMI and it's into my tv, the update was for my nvidia GeForce graphics card, if you need anything else just ask, I'm not 100% on what else to list