SMEG Fridge Freezer FA350X

Hi there
I have just moved into a rental property with a SMEG Fridge Freezer which I think is model FA350X. The alarm started going off today after I opened the freezer door, which is usually the sign that one of the doors is open. I made sure that both the fridge and freezer door were closed properly but it kept ringing. I muted the alarm and checked the temperature of the fridge which is at the recommended 2 degrees and the freezer was at the recommended -20 degrees C. However, when I press the freezer button the LED screens comes up with the letters AL and the red alarm display button is illuminated. I know that it is therefore an issue with the freezer and I have noticed that the temperature has dropped as the bread in there is slightly soft. The door seal seems ok. I just wondered what has caused this drop in temp? There is a large build up of ice at the bottom of the freezer which I will defrost next but could this have anything to do with the problem? In the meantime, I have increased the temp in the freezer to -25 but the red light is still on. I have also tried switching the fridge on and off but the alarm just starts up again and the AL sign starts flashing on the screen.