If you use to have cold ac, but now you don't, the simplest and most common problem is a small leak in the system and loss of freon. Recharging the ac system will rejuvenate your ac until the freon gas leaks out again. When the technician performs this service make sure they have dye in the system to locate a small leak. Other testing may be necessary to determine if the compressor and other ac components are working properly.
Make sure to note if they system makes any unusual noises when trying to operate. A visual inspection will only enable the technician to disclose any major ac system defect, such as a broken or missing hose or belt.