I have a Dell 8500 that I recently upgraded from Microsoft Windows 8.1 Professional to Windows 10 Professional. The upgrade went smooth but after a few days the system would only boot with a resolution of 640x480. The AMD Radeon HD 7700 is connected via DisplayPort to a Dell U2312HM monitor.
I tried every solution that I could find on the Web and still could not resolve the issue. I did a complete uninstall of the latest AMD Windows 10 Drivers. Since the resolution was stuck at 640x480 I was unable to re-install the drivers.
Based on a suggestion from one Windows TenForums I was going to attempt to use another connections on the Dell U2312M monitor. This monitor has a DisplayPort, Mini Display Port and a DVI.
I unplugged the DisplayPort Cable from the AMD Radeon HD 7700 to get a better look at the available connectors. The video card has a DVI, DDMI and DisplayPort. Since I did not have a long enough cable to go to the monitor I plugged the DisplayPort cable back in.
When I re-started the computer the resolution was back to the 1920 x 1080.
I checked with two other friends that were having similar issues with a Dell and HP computer stuck at 640 x 480 and removing an reinserting the DisplayPort Cable. I suspect I would have had the same results if I powered down both the computer and the monitor. In all case the monitors and computers had not been powered down at the same time.
Hope this helps someone.