Room air conditioners draw a lot of electrical power. If your air conditioner is not turning on, check for a tripped circuit breaker or blown fuse caused by an overloaded circuit. Also check that the thermostat to be sure it is set to cool and to a few degrees below room temperature.
If your air conditioner doesn't cool, you may need to clean or replace the filter, which might be restricting air movement. If that doesn't do it, clean the evaporator and condenser coils.
Room air conditioners can sometimes become very noisy. Check that the fan blades are not bent, and clean them. You may need to tighten the mounting bolts for the blower, which could be rattling in its casing, and lubricate the motor according to the manufacturer's directions. Also check that the unit's mounting bracket is firmly connected to the window or the wall; it too may be loose and rattling.
If you are working on a room air conditioner, clean the drains that allow condensed water to drip away from the unit.