Why won't my hot cold temperature knob in my 95 Dodge Ram turn?

If I recall correctly, on those vintage vehicles with base level HVAC system, they were still using cable controls for temp control. Most likely you have a frozen cable or a stuck blend door. Dodges have always had issues with blend doors breaking inside the HVAC box. This is a relatively major repair which requires a lot of the interior to come out and mostly best left to the pros, or very advanced DIY person with a lot of patience and time. A cable can be a pain but not quit so bad generally. The last item I would expect but possible is the control head has bound up in some way, but is possible. Good luck Clint