Why would my air handler stop working on the auto fan setting?

I noticed that my room was getting extremely warm. When I check the vent, no air was coming out. When I want outside and found the condenser unit running I immediately came in and turned the thermostat fan setting to on. The air started coming out cold, so I turned the thermostat up to 90 to get rid of any potential
Ice/frost that may have accumulated on the air handler coils.

So now my a/c works only when the fan is set to on.

Is it my thermostat that went bad or some sort of rely in the air handler?