Not having audio coming from a microphone could be caused by a few different scenarios.
Firstly check that the transmitter and receiver are both on the same frequency as this is key to the system working.
If you are not seeing an RF level on the receiver check the RF MUTE on the transmitter to make sure that the RF MUTE is OFF.
Next check to see that there is audio activity at the transmitter. You can visually see audio entering the transmitter (bodypack or handheld) by looking at its AF meter.
If no audio is seen make sure that the microphone element is securely in place. If using a handheld make sure the capsule is securely tightened because if the capsule is partially untwisted from the body of the microphone the connectors won't line up and that will mute audio. If using a body pack transmitter make sure that the microphone or guitar cable is securely in place by disconnecting it and reconnecting it.
If using a body pack transmitter (or a handheld with a mute switch) make sure the "MUTE" switch is not accidentally engaged (either fully or partially). The best way to do this is click the mute switch on and then back off.
If you are seeing "AF" activity on the transmitter but still no sound from the receiver it is most likely an issue with the "PILOT" tone setting. Ensure that the PILOT is active (or inactive) on both the transmitter and receiver. You can find this setting in the "Advanced" menu. NOTE: Sometimes the "Pilot Tone" will read active but is is not correctly engaged so it is good practice to turn the "Pilot Tone" inactive and then back active to reset it and ensure it is engaged correctly.
NOTE: If using a ENG system (like the EW 100 ENG G3) make sure that the body pack transmitter and body pack receiver are in the correct positions. The two body packs look similar and are sometimes mixed up (ie: the microphone is attached to the receiver rather than the transmitter and the transmitter is connected to the recording device/sound system). A good way to remember the correct configuration is to remember that the body pack transmitter has the MUTE switch because you want to be able to mute a microphone.
If you are seeing RF and AF levels on the receiver but do not have any sound through your system it would be recommended to check the cables between the receiver and sound system, check the inputs on the sound system and check the AF out level on the receiver to ensure that it is set to the appropriate level for your sound system.