ac and heater wont blow air out of vent on 1999 Ford F-250 Super Duty

can hear blower motor work but no air will come out of vents

Asked by for the 1999 Ford F-250 Super Duty
Question: does it blow out the defrost or floor? If it is just blowing out of the defrost, a vacuum leak may be the cause. Fords are designed so if a concern happens in the air distribution system, it defaults to defrost only. This vacuum leak could be under the hood or under the dash. Start looking where the vacuum source goes into the cab on the passenger side of the firewall.
does not blow out of floor or defrost