Center Vents Not Blowing Air

My car center vents are not blowing air, only the two side vents are working. Be it on A/C or on Heater mode. I already changed both the cabin and engine air filters but it still not blowing any air. What could be the problem? If in case, how much will it cost me to have it repaired? Anyway, I have a 2015 VW Passat. Any help will be appreciated, Thank you in advance.