Bauknecht Under Counter Freezer on the blink

Hi I hope someone can help. My husband noticed the other day that our freezer (Bauknecht Under Counter Model 855205015020, over 12 years old) was sticking out a bit from the other units and nudged it back in. When I got home at the end of the day the high temperature warning was sounding, so I checked the door was sealing and set it to superfreeze. When I went to check turn superfreeze off the warning was still going, and the high temperature light was on. The next day it was the same, and clearly starting to defrost.
I have spoken to an engineer who said it might be the doorseal, the compressor or the thermostat.
As I said, I think the seal is ok (you need to give a definite tug to open it), & it sounds like if it’s the compressor I need to buy a new one anyway. Is there any way that the thermostat might have become dislodged? Would it be really hard for me to check?
It seems sad, and (hopefully) unlikely that it’s given up the ghost after so many years of service from just having been nudged! I hope some can shed some light. Thanks in advance.