Okay, I can fix most anything, but I know next to nothing about HVAC. A friends got a central air conditioning unit that the compressor went out on, which was fine as it wasn't hot. But now it's cold, and the heater will not come on either. It is not a gas heater, and I can find no elements, and am wondering if maybe this is a AC/Heat pump? If so, would replacing the compressor work? I guess that's a pretty obvious answer, but... They do not have any paperwork for the unit, nor do I know what kind it is, but am going to take apart the condensing unit so I can get to the compressor and check it out, size style, make, model. Anything I should know? Or look for?

OK first if it is a heat pump u can tell by the lines going to the compressor! There is a sencor as well valve that has three lines coming from it! Either way if your compressor shot you need a new condensing unit, its better to buy a new one than to try to put a compressor in an old one. If not your going to have a lot more problems than just that! And its not much more to just buy a new one

If it is a heat pump, there should be electric elements for backup heat. Or gas backup. Replacing compresser means you would have to be licensed to work with refrigerants. Probably time to call for professional onsight help. Will save you a lot of headaches in the short and long run.