The unit can be serviced and recharged. However you'll need to find someone who knows what they are doing because there aren't any service ports to charge it from like on a car. I'd go with the cleaning first and try that, if not, it most likely will need a charge (or worse). It should take R22, which is still manufactured and readily available, to anyone qualified to purchase it.
Don't get to deep into fixing it though, it may work out better to just replace the unit instead of trying to fixing it if it's old.
Bottom line: Have a professional look at it and go from there.