Hmmm.... Have you tried uninstall the video device from the device manager, and remove its driver as well?
The shutdown, restart PC
Would be weird if both cards were broken... Is the DP cable and monitor connected to the card in this situation? Maybe the DP cable or monitor has an issue and feeds back the problem to Windows? Easy check, disconnect it
If you have a spare harddisk or SSD, I'd connect that, and install W10 freshly on it. Should be fast, and it will then tell you whether it's a Windows issue or a hardware issue.
Similar to what Crash stated: you could try and run Ubuntu from a USB stick, and see how that works - with DP and all ...