Over the winter, my Air Conditioning ceased to work. The switch is fine and the fuses have all been replaced for good measure. I had the compressor checked and was told that it was not the problem. I have narrowed it down to having no freon (or whatever they call it nowadays) or the compressor. Any ideas?

If the amount of refrigerant in the system is too low, there is a pressure switch which will prevent the system from operating (since the lubrication is carried in the refrigerant ...operating a system without refrigerant will cause the compressor to self destruct due to lack of lubrication in short order, hence this switch). In all liklihood you have a slow leak somewhere and are just low on refrigerant.

Concur with Glacier991. Your type of refridgerant is now your immediate concern. If it is R134a you can just add a can or 2 and see if starts working. If it the older R12 loaded unit you gotta problem. But there are several good solutions. Let us know.

It uses R12. But I know that I can upgrade to 134a. What's my next step?

well a couple. First take it somewhere let them charge the system.... and leak check it.... the second would be to do a DIY conversion. There's an entire long thread on that subject in useful threads.

Yes, presumume have leaks fixed you can just pay for an R12 refill. Very costly. Last one I did we refilled with an R12 substitute called "Freeze 12". Also added a judicial quantity of make up oil. It worked out well. Have never done an R134a conversion.

There are alot of alternative refrigerants on the market that are compatable with R12 systems and require no conversion, the big problem with these are is that most of them contain propane... not a nice thing to have in your car if there's a fire.

Eneurb is right. In the VERY long thread on converting in the Useful Threads forum, I discussed alternates and their downsides, even posting the constituent parts of each of the "replacements"... Also, remember....some shops won;t work on them if you add that stuff... for good reasons I discussed in that thread. My advice ... still? Stay R-12 or convert to 134a.