355 air con problem

i went to see a client yesterday morning and parked my car in their outdoor carpark for about 2 hours

when i came back to collect my car it was hot (but acceptable) cos it has been sitting under the sun for roughly 2 hours

so i start the engine and the air con but it wasnt cold!!! it was working perfectly fine.....

what could be the possible causes?

guess i might have to charge my client for repairing my air con