Is there a monitor driver to install from the LG website or did the monitor come with a CD?
Check Device Manager to make sure that the monitor is not set as disabled. If it is, enable it and try to restart the pc. Sounds strange but Windows 10 disabled my monitor once in Device Manager which stopped FreeSync from working.
If that fails i would try reconnecting your DisplayPort cable to the GPU and then restarting the PC.
I've installed the LG driver as soon as I got the monitor, which was a while ago. I checked the Device Manager and both of my monitors are currently enabled.
I've also reconnected the cable and restarted but alas, it didn't work.
Both of my monitors do work though, it's only Freesync that is not working. I took a screenshot which displays my issue:
(Display 1 is my Freesync monitor, Display 2 doesn't support it)
I'm having the same problem since updating. I have a BenQ XL2730Z and a Sapphire Radeon R9 390X Tri-X 8GB.
Tried what amdmatt suggested to you but had no luck.
Hoping this is just a bug that will be patched soon.
Got some help from another forum and fixed it!
Has to reset the monitor using it's OSD.