Probably the biggest single failure in air conditioning units is loss of
refrigerant charge.

After that, there is a long line of defects which can cause a failure to
cool properly.

If you dont know what you are doing, you are probably well advised to take
the car to
a competent mechanic.

If you want to buy a charge kit with a simple pressure gauge, you might be
able to
"top up" your refrigerant charge, and it might return the car to normal
cooling, for a while.

A kit like this is probably less than $25.   It is a simple task, but if you
dont know what you
are doing, you can spend your money and still not get a fix.  At worst case,
you can hurt
yourself.