If the cold air has ceased to flow from the vents of your Ford Focus it probably means the A/C unit needs to be recharged. You have the option of bringing the vehicle to a dealership for repair, but that can be very costly. Instead, you can buy a can of R134a coolant that comes with its own hose and charge the A/C unit in your Focus yourself. This can be done pretty easily.
Consult the owner's manual to locate the area where the service fittings lie on the vehicle's A/C. This location varies depending on the model of Ford Focus you own, which is why you must consult the owner's manual for exact details. You can locate the high-side fitting on the hose that goes from the compressor to the condenser. The opposite fitting is located on the suction hose that runs from the accumulator to the compressor.
Attach the charging hose to the coolant can.
Turn the can's valve. This places a hole on the top of the can to let out a tiny portion of air and refrigerant. When you finish, close the valve and attach the service hose to the side fitting on the bottom.
Start the car engine and place the car's A/C fan at its strongest level. Turn the valve of the coolant can to let its contents into the A/C unit. Repeat this step until the air blowing from the vents feels cold and the vehicle interior is sufficiently cooled.
Proceed with caution because your vehicle has a pressurized A/C system that could explode if you fill it up too much. Note that you need to take any left over coolant to a repair shop. This is required by federal law.