A/C decided to pack up today, in probably the hottest day of the year

.
The control light for the aircon is illuminated and I hear a clicking noise ( i assume this is the compressor) clicking in and out but yet the air is tepid and not in the slightest cold.
Any ideas? Does it need a refill? I thought the compressor would not function if the refridgerent gas was low??
Thanks.
M