A few thoughts. Be sure to test the video monitor on both the GPU and motherboard video connections.
If that fails, remove the GPU and try the onboard video system.
Some months ago, I built my first computer. I got it set up, and for a while, everything worked fine. After leaving it off for a few days, I ran into a problem where the monitor would not recognize the input. Some searching online, and someone said to disconnect the graphics card, and plug it back in. (Computer was off, of course). I did this, and it fixed the problem. When I moved the computer, I ran into the same problem, and the same solution fixed it. A few days ago, I disconnected everything so I could get it plugged into the battery backup, and I had the same error, only this time, the graphics card thing did not solve the problem.
I built according to the below guide: https://pcpartpicker.com/guide/GFLrxr/entry-level-amd-gaming-build
Though I chose my own case and monitor.
Has anyone else had a similar problem, and if so, how did you solve it?