Hi Joe,
Well the good news is that should make it easy to figure out.This morning I did test with the arrow buttons and it failed intermittently more time than I could keep track of, now the machine is down.
One possibility is a bad motor cable. Sometimes cables that continuously flex eventually fail in an intermittent manner. If the cable momentarily breaks while moving it can cause a stall.
Another possibility is that the axis load increased due to something like binding and the motor torque is insufficient to avoid a stall. But normally the motor would then stall and vibrate rather than being silent.