Generally, if your brake has stopped working entirely, you have an electrical problem. This is most likely a failed switch (probably a fuel pump or clutch switch). You can usually confirm this by hot-wiring the brake, bringing battery power directly to the solenoid leads (with the engine at idle).
Be sure to disconnect the existing leads to the solenoids so that you do not damage your control system.
If the brake does not operate when hot-wired, you may have a solenoid failure. If the solenoid is actuating but the brake is not turning on, the problem is probably mechanical.
Your engine brake dealer should be able to help you locate appropriate troubleshooting information for your engine brake.