I appear to have solved this on my system after a long struggle. As it took me some time, it looked like a good idea to me to post this.
I have a Dell XPS8700 with an NVIDIA GTX645 card with two Dell U2414H monitors attached through HDMI and DisplayPort. The one on the DisplayPort suddenly switched from 1920x1080 to 640x480 (really small! ) with no option to change this.
This problem happened with Windows 8.1. After a fresh install of Windows 10 it was solved for 2 weeks, then it started all over again.
There was a hint in the NVIDIA control panel: it did recognize the Dell monitor through HDMI, but through DisplayPort it was something like "Nvidia Digital Monitor" - or, "I don't know what it is".
This is what I did:
1. I updated the BIOS to the latest version.
2. I updated the Chipset drivers for my system
3. I installed the latest, greatest drivers from NVIDIA
4. In the settings of my monitor in the menu DisplaySettings - DisplayPort 1.2, I ENABLED that feature
I think step 4 did the final trick for me, possibly in combination with the rest)