Update your post to include full system hardware specs and OS information.
Old monitor, new monitor? Connectivity? Try another known working video cable matching the connectivity between GPU and new monitor.
If ShadowPlay originally worked then I would suspect that the software has become corrupted.
Test the old monitor again - determine if Shadow Play still works.
If not then reinstall Shadow Play.
If Shadow Play does work then compare specs and configuration settings between the original monitor and the new monitor.
Do be sure to check the new monitor on another known working system as well.
Check the new monitor's manufacturer's website for any available drivers.
Objective is to narrow down the source of the problem. Directly or by elimination.