Sharp AF-M808X air conditioner. suddenly there is only fan air movement. there is no longer any cooling; turning the thermostat knob to a higher level doesn't "kick in" the coolant. but nothing else seems out of the ordinary. this morning it seemed like the cooling was sluggish so i cleaned the air filter.

Simple test: the thermostat in your unit is electromechanical - unplug the unit from the mains, take it out and measure the resistance between the 2 electrical contacts, if it doesn't goes to near 0 when you turn it even a little bit then it's defective and it will have to be replaced.
If the resistance does indeed goes to near 0 then the defect is in the AC system. Another simple test: open the casing of the unit and put your hand directly on the compressor; now turn the thermostat button. If it doesn't vibrate then the starting capacitor of the compressor has failed and it will have to be replaced with a similar one.If it vibrates then the cooling circuit has got a leak and all the fluid has been lost. For that you will have to call a pro that will solder the leak shut and refill the circuit with the appropriate fluid - only he will have it, it's not sold on the customer market.