There are various reasons as to why your aircon is not cold but here are some of the possibilities:
1. Internally clogged with too much dirt and dust
2. Your system is short of gas.
3. Capacitor is faulty.
4. Thermistor faulty
5. PCB (Printed Circuit Board) faulty
There are numerous other issues that your aircon could be facing such as a gas leak etc. Hence, it is always important to let an experienced technician conduct a proper assessment before any job is done so the right problem can be fixed with the right solution.