Due to the fact that the bulk of the unit hangs out of your window and is exposed to the elements year round, window air conditioners have a greater tendency of breaking down when you need them cooling the most.  Now, being the air conditioner guy that I am, you would expect me to tell you to call out your local AC repairman because it is usually an easy fix.  However, with labor prices going up and up, and the price of window air conditioners coming down (about $300-$400 for a descent air conditioning unit) the right move may be to scrap the old junker and order up a new one, especially if your existing unit is outside of any manufacturer warranty period.

If you run the numbers and decide that buying a new unit is the thing to do, then here is a way to cut your costs even further.  DO NOT THROW YOUR OLD UNIT IN THE TRASH.  You need to take it down to your local scrap metal place and cash it in for around $50 worth of Copper Tubing that is inside of the unit.  You will get the most money form the scrap yard if you remove the copper from the unit yourself, otherwise they will factor their labor into the the overall value and reduce your take considerably. Another thing that I would advise is to call around to find out what they are paying for copper before hauling it down there and compare that to the price of copper on the open market.

Most scrap metal places will pay you cash on the spot.  Now you just cut $50 off he cost of your new unit.  Either that or you can just pocket the found money and not tell your spouse that I told you about it (-;