Fridges use a gas. If you look at the back of the fridge you will see what looks like a radiator and a pipe system leading to it from the compressor. If what you have been told is correct there is a leak in this system. The fridge will need re-gassing. You will have to get someone from a local shop who repair fridges to do this for you, however before re-gassing the fridge they should first check the system through to find out where it is leaking. Try to find a local family business rather than one of the large national retailers as local small business's usually give more reliable service. You may find one of the major retailers will try to tell you that you need to buy a new fridge (from them,
of course!)