Distance yourself from your sound system's monitors, if you experience feedback when using your lapel mic. If your lapel mic gets to close to its output speakers -- like any other microphone -- your lapel mic will generate a loop of feedback.
Check the battery LED on your lapel mic transmitter, if you notice a gradual increase in the amount of static present in your monitor speakers. Ensure that the LED is lit bright green -- if the LED's light is dim, you'll need to replace the transmitter's battery. Also, moving closer to the microphone's receiver unit can correct some static issues.
Check the volume level on the microphone's receiver and transmitter units, if you lapel microphone's output volume is low or there is no sound at all. Check that the transmitter unit is switched on and that the unit's battery LED is lit. Also check the volume level on your mixing board. If necessary, raise the volume on the mixing board channel to which your lapel microphone's receiver is connected.