Since v9 or so I've had this issue, but it's gotten worst with v15 (currently on 15.0.0B.043, but it's always happened) . I use three monitors, with Resolve in dual monitor mode. I have occasionally walked away with it open and had my monitors fall asleep. Upon waking them, resolve has collapsed onto one monitor and shows me only the secondary monitor.
While annoying, I used to be able to use menus to go into single monitor mode, then back into dual monitor mode and it would restore itself. Now, however, all menu functions don't work. I have to quit out of it by right clicking on the icon in my windows tray or use the task manager.
Yes, I realizing turning the sleep mode off my monitors would fix the problem on the user end, thanks for asking.

Again, while this has happened across versions and several system set-ups, my current system is:
OP: Windows 10 Pro 64-bit
Chip: Intel I-7 7820X 3.60 GHz
Ram: 64 gig
MB: ASRock X299 SLI/ac
GC: NVIDIA GeForce GTX 680
LG HD monitors
Lots and various hard drives
Thank you!