You pretty much covered the basics. It sounds like the blend door is stuck in the closed position. Blocking off the flow. Look close and listen as you switch from hot to cold on the control.
The door makes a pretty obvious thump when it closes. And it's not that uncommon for the linkage to come off or break, either of which could lead to no heat.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.