Not a lot of details.
Do you feel any tension while moving the knobs?
Do you hear any trap door movement?
May have snapped a nylon string that actuates the different doors. There are 2 of these, one moves a trap door from cold to heat and the other moves the trap doors from defrost to different lower position as the little icons show. These nylon nerves have a tendency to break when people attempt to change them before any heat has flowed through the system long enough to thaw out any possible frost that may have frozen these shut.
Here is a sample, easy job but you need a lot of patience and time.