thermostat could b bad or the heater control valve.........those are the 2 most likely....next would b heater core clogged ...next would b the door mechanism is broke or non-functioning....its either a cable or a vacuum tubing set up.........could also b the control panel is bad........
so trouble shoot in that order......
*o yea btw if ur water to anti-freeze mixture is not correct it can make it cold....but that's not very likely
without other problems like a frozen block
**if u just got this vehicle heater core may b by-passed cas it leaks......but i doubt it...