As Lee stated...check above...hopefully, it will be simple, such as the heater control cable (that sits on top behind the fan unit under the hood) may have become disconnected...simple loop in end of cable that goes back over a small post on the valve actuator arm...just saw this on a new 2000 that a friend brought over 2 weeks ago.
Also, be SURE that ALL climate controls turned to RIGHT.
If coolant OK,and above checked, and still no heat...BE CAREFUL...and suspect thermostat.
Let us know.