I assume the front blower is still working, this is you still have air going through front vents.
First, I would check the front air flappers. These are controlled by vacuum. Maybe you have a vacuum leak and the flapper are not switching to the evaporator side. They might be stocked on the warm air.
Second, check if you hear a low hizzy noise. This is made by the expansion valve. This valve is responsible for creating a pressure drop on the refrigerant fluid and creating the low temp on the evaporator to cool the front of the car. When this valve is bad there is no cold temp created in the evaporator.
Hope it helps.