First check that the AA batteries in your handheld microphone or belt pack are good. You can quickly do this by turning on your mic or belt pack and noting whether the red LED light flashes once and stays on, indicating good batteries. If the light does not flash at all or continuously flashes then the batteries need to be replaced.
If your microphone is still not working, double check that you have matched the frequency number on the mic or belt pack with the same frequency number on your sound system.