Just to add more depth and help troubleshoot this more, I tested a few different things.
Button works fine, both on keyboard and arcade input. Notepad, MaLa config, and running a MAME game all recognize '1' when pressed.
Oddly, my MAME game list has been changed somehow; I had all filtering disabled, but now you're forced to go through a filter menu when you select MAME from the emulator list. I've double-checked the config, and all the filters are set to "Off". This seems to have happened at the same time as the other emulators stopped functioning.
I've also checked the MaLa log, and the only indication therein that I'm trying to start a game are the Info lines about videos/pictures not being found. I've checked a log from when all was working fine, and those are still there -- so clearly, whatever's failing isn't being logged.
I'd be happy to provide any other information to get this figured out... my users love our cabinet and are begging me to get it working again!