Vents not working

Audi A6 Quattro wagon dashboard vents do not work, neither hot nor cold. The top vents work but nothing else. Any suggestions?

I wonder if these vents are vacuum controlled? A lot of cars use them, domestic and import. If so, the defroster is the default mode if vacuum fails. Look for a vacuum line from the engine to the firewall that feeds the climate control system for breaks or leaks.