I have a 99 GMC Envoy and the heater is not working. One heater hose is hot the other is cold? Is there a valve on this model?
I don't find a valve for this model in any parts lists, so it is likely your heater core is blocked/plugged, or the coolant level is too low to properly circulate.
First, check that you have enough coolant in the system.
If there is enough coolant, then there are a number of ways to try to clear the heater core. It is up to you how you want to attempt it. Flushing it directly would probably be the most likely way to know for sure how bad it is.
Here is one way ChrisFix does it.