Well, I suppose it could be low on refrigerant by developing a recent leak. It could be the reversing valve is stuck in mid position or something else in the refrigerant circuit.
When you get into those things, you really need to have a tech or at the very least the tools to check the refrigerant charge and system temps with.
If the outdoor fan is running, this tells me there is indeed a call for cooling from the thermostat
. It tells me there is power at the unit. It also tels me the contactor is pulled in and not tripped on a high or low pressure switch. If the compressor is indeed running, it's not pumping or there is an issue with the refrigerant charge or refrigerant system anyway.
I assume your indoor fan is blowing air through the vents but its just mild, not cold. (if it was cold, the air coming out of the outdoor fan would be warm.
You see Air conditioning just moves heat. It removes heat from where you dont want it (in the house) and rejects it to a place that doesnt matter (outside)... This was the best definition because it's descriptive.... Willis Carrier
I said that to say this, you cant remove heat without rejecting it. If both are mild, there is an issue that will ultimately require service.