Could be the blend door not working, or an issue with the heater core.
If this is the first time in quite awhile that you have turned on the heat it is possible that the heater core is blocked up. A heater core flush is super easy to do. Disconnect both input and output of the core. A garden hose to the output of the heater core and run it for a few mins, then switch to the input and do that. flop back and forth a few times if it is not flowing well.
Or there is air in the heater core, something that will need to be addressed after a core flush anyway.
To purge air in the system park pointing uphill or jack up the front end and with the radiator cap off run the engine. You need keep the radiator topped off, and do this until you get hot air.
NOTE: the clear bottle of coolant is not where you add coolant to the system. That is the overflow and will only function if there is enough coolant already in the system. Add it directly to the radiator. Don't open the radiator cap of a hot engine.