I had trouble with my spindle control, which works on the PWM through a Digispeed to an Omron inverter. Probably much like yours.
The only way to check that Mach is putting out the correct PWM signal is to check the waveform with an oscilloscope, and try various speeds and see the waveform change.
If yours is a sudden fault, either your CNC4PC card has gone, but the other thing you might check is the pulley setting on Mach and the table of speeds for it.
As you probably know, you have to select a pulley, and write in the maximum and minimum speeds for that pulley. Mach will not set speeds outside that setting.
Make sure they have not altered.