Some possible causes:
- the cable from the heater control to the valve under the hood is disconnected
- the valve under the hood is seized and won't open
- the heater core is plugged and won't allow coolant to flow through it
- the coolant is low and there isn't enough to get to the heater
- there are leaves or other debris preventing air from getting into the heater core
- the ductwork under the dash is damaged or disconnected

Low coolant is the easiest to fix, so I would start there. While under the
hood, have someone operate the heater control so you can identify the
valve and check it out. Under dash work starts with a visual inspection.
Having the fan operating on high will help determine if there is air
blocked or leaking somewhere. You should be able to see the controls work.

Dan

(This account is not used for email.)

Dan had some good advice, the only thing I would add is to check your
heater box under the dash. Most new cars heater boxes are controlled by
vacuum. You could have a vacuum leak, a disconnected hose or something
else causing the temperature control within the heater box to
malfunction. It could be a fuse. I would check them all.
Good luck