Below is an example of damage to the cadence sensor module. This can happen when the rear wheel is removed and reinstalled incorrectly. The smallest cog (11T cog) has made impact with the cadence module which effectively damaged the outer layer of the cadence sensor module.
The result will be an unresponsive cadence sensor. In some instances the sensor can have an intermittent or surging behavior. The solution is to have the sensor replaced.