I think that Diplomat is a brand of the now-defunct MFI.
Please take on board that i'm NOT a professional Whitegoods engineer
If this is a built-in fridge, the door-hinges wear/break , the door doesn't seal properly, the light can stay on, thereby heating the interior as fast as the compressor is trying to cool it.
Again, instead of the big, black ,louvered plate at the back (the condenser) it's quite common to have a small "radiator" hidden underneath with a fan.....
these units suffer fan-failure and choking-up with fluff.
you need to check if the compressor is actually cycling on-off...if it's working continuously, either you have compressor valvefailure/gas loss OR, if the "cold bit" is actually getting cold, that would point to the excess fluff/cooling fan failure.
In any event, it doesn't hurt to clean the fan/fluff etc.
You didn't say if the thermostat is actually allowing the compressor to run.
No compressor = no cooling.
OK before I get shot down, there are ABSORBTION fridges and also PELTIER-EFFECT (cold-boxes for sure, but is anyone making a Peltier 'Fridge???
or, come to that, absorbtion, other than for caravans/boats?
old enough for wisdom, young enough to be compos mentis.