its because the resolution the graphics card is trying to output isn't supported by the monitor.
this might help;
Do you have a win 10 installer? I only need it to be a boot drive. If you don't, on another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
Now lets see if we can boot into low resolution mode so you can set the GPU resolution toa better one.
boot from installer, on 2nd screen after languages, choose repair this pc, not install
choose troubleshoot
choose advanced
choose
start up options
hit the restart button
choose option 3) Enable low resolution video
PC will restart with low res settings and hopefully you can fix current display settings.