I use to top off my old truck every spring by using the low side charging gauge and hose from the auto parts store. Once I started doing more mechanical work on cars for friends and family so I decided to learn more about AC systems. I am by no means an expert but I have learned that low side gauges do not give you a good diagnoses. You really should be checking both high and low sides of the system. Check out some youtube videos from eric the car guy. I believe he has a few on AC basics.

Start with petrified.rabbit's advice and see if the the compressor is running. I normally hook up my gauges and look for pressure in the system. If the compressor starts but kicks off you maybe low on refrigerant. You will need to watch both sides of the system to see what is dropping first, high or low side. If high side drops first you are low on refrigerant.

If the compressor dose not come on at all and you have some pressure in the system, then try bypassing the low pressure switch and watch your gauges for high and low side pressure. If the pressure climbs up then drops off, then repeats you maybe very low on refrigerant. If the pressure climbs high on the high pressure side and cuts off you have a blockage or metering issue.

Remember the correct way to charge your car is by weight not pressure or temperature. Low side pressure gauges may get you cooling but the system will preform best when charged to the correct weight. Charging from a low pressure gauge is like an educated guess. It may get you close to the right answer but not always the best answer.

Many times AC not running issues have nothing to do with charge and are sensor or relay issues. Poor performance can be from clogged filters and coils, broken air blending doors or failed cooling fans. Good luck.