Unlikely to be a system resource issue, since the PC is running a quad-core i7-6700K CPU 4GHz with 32 GB of RAM and has been rebooted multiple times. I am running Windows 10 Pro 圆4 v1809 OS build 17763.379, fully updated as of this morning, with US English as the only installed language. I am unaware of any specific software installs or system updates that presaged this problem. Over a year and CTRL-Z worked fine until a few weeks ago. To my knowledge, no other program on the computer uses CTRL-Z as a shortcut or hotkey. Other keyboard shortcuts that I generally use (and I use a lot of them) still work fine, like CTRL-A, CTRL-C, CTRL-V, CTRL-W, CTRL-S, etc. To be clear, the Undo function still works fine if selected from the menu or shortcut, but the keyboard combo CTRL-Z does nothing. It doesn't work in any application or on the desktop. My Control-Z shortcut for undo stopped working a few weeks ago.