Vents blowing room-temperature air when your air conditioner should be running could be a sign of potential thermostat issues or problems with electrical components. Most causes of this problem are generally inexpensive and quick fixes with the help of a professional.
The first component to double-check is your thermostat. Sometimes settings can be changed that turn on the fan but not the compressor, so while your fans are blowing air, your air conditioner itself isn't actually running. Make sure your thermostat is set to cool regardless of what the fan settings are.
Low or dying batteries can also cause issues. Even if your thermostat is displaying something on its screen, that doesn't necessarily mean it has enough power to operate the fans and air conditioner. Double-check your batteries to look for any signs of corrosion, or test them to see if they are low on power.
Your air conditioner's fuses are responsible for preventing electrical damage to expensive components in your unit. If they fail, your air conditioner will no longer get the power it needs to run. You may hear a humming sound from your compressor, but the fan won't actually be moving.
If the problem is your fuses, it's a relatively quick and inexpensive process to replace them. You can also test them with a multimeter before replacing them in order to verify that they could be the source of the problem. If you do opt to replace your fuses, remember to take precautions like shutting off your circuit at the breaker to ensure you don't get injured and that no damage is done to your system.
In some cases, the problem could lie with your air conditioner's capacitor. The capacitor is responsible both for providing the jump start necessary to get the air conditioner fan moving and also to keep it running. If this capacitor has failed, your air conditioner won't run even if everything else is working correctly, and the air coming from your vents won't be cooled.
Since your air conditioner will still be receiving power, you may also hear a humming sound similar to the one caused by failed fuses. Capacitors are moderately more pricey than fuses and should be replaced by a professional, but the good news is replacing your capacitor should completely solve your problem.
If you suspect that any electrical issue might be the problem, it's a good idea to turn your air conditioner off and avoid trying to use it until it can be repaired. Otherwise, your system could be strained and be damaged further. Contact an AC system service for more information.