There are a few things to check.
If the unit is equipped with a controller, check temperature and humidity level settings.
If the intake air is not within the preset range, the fan will not turn on. Either adjust the settings or wait for the intake air temperature and/or humidity to change.
Check that the unit has power. A circuit breaker may need to be reset.
Turn off power to the unit and make sure all electrical connections are correct or have not come loose.