I recently changed to mach4 from mach3.  I got everything setup and working yesterday.  Today I ran the softlimits plugin and edited a script so that soft limits are enabled when starting mach4.  Yesterday z probing worked.  Today the motor doesn't move at all when z probing.  Even running G31 in the MDI gets no movement.  I can jog the machine around with no problem.  The indicator light on the Diagnostics Tab is off and if I manually touch the pad to the bit, the light comes on.

For what its worth, I can open up my old setup in Mach3 and the G31 command works fine.