It is within the warranty period
If the product as been installed in accordance to the fitting/wiring instructions and there is a supply at the fan but it does not work, then return to place of purchase for a replacement.
It is outside the warranty period
Check there is a stable supply to the fan and that the impeller is able to spin freely and there is no blockage within the fan and any duct run or termination.
Check nothing is obstructing the shutters mechanism