Most certainly unrelated, but I've always had trouble with the MDI box going away after clicking in it and typing. This across about 5 computers in the time I've used Mach3, about 4 years. I've learned that if I hit ENTER when I want to MDI, the box will always stay up, so that's what I do. If I click in the box I might get 4 or 5 characters or I might get one, and the box just disappears, but wait, not always!! I've posted this over the years, but it is just me, I guess. I've tried to tie this to some action, but can't seem to isolate the behavior.
You mention a button on your MPG. If the MPG uses a plugin, like VistaCNC pendants or perhaps others, disable it temporarily and see if MDI works. I use VistaCNC pendants and once in a while I've had things act abnormally with no explanation and I sometimes suspect the plugin, but I can't say I've ever proven it!!
Other than that, I know of no reason why MDI would stop working completely either
