There are a few factors that can cause this:

• There is a leak and the system is low on freon.
• The system is overcharged. Too much freon is not good either.
• Some part just broke and it needs to be replaced. AC compressor might be broken or expansion valve / orifice tube might be clogged.
• There might be a problem with an electronic part. Usually these problems are harder to pinpoint and the diagnostic might be quite expensive.

In order to prevent possible AC problems, remember to use it for a few minutes every week, even in cold weather. This will keep the o-rings and the other parts lubricated.