HELP!! Diplomat Fridge is showing 'H' on the thermo!?

Hi
I have a Diplomat Fridge Freezer model APM6855. It has been working fine up til recently when all of a sudden the fridge thermostat read +8 degrees and everything is going off! Now it is showing 'H'!! Please help as we can't afford a new fridge and I haven't got the instruction manual. It has nothing ontop of it, it has good ventilation at the back and sides and the freezer is fine. Thank you!