How can i fix My White Knight WK245A Freezer?

My freezer decided to stop freezing last night, the freezer red warning light is on and even after letting it defrost fully it's still on,
The manual explains that the red light comes on if,

A) The freezer has been switched off for too long,
B) The door to the freezer has been open too many times,
C) A large amount of foodstuffs has been loaded
D) The Freezing system has failed.

The compressor is still working, but its not getting cold.

Any help would be appreciated.

Thanks