I have exactly the same problem with my Bosch Exxcel Fridge Freezer. One of my children left the door open about 6 months ago.
Twice now, ice has built up in the cooler fins which has pushed the plastic cover containing the fan.
It is fairly straight forwards to dismantle the back of the freezer, to clear the ice and check
Firstly having turned the power off, remove the plastic facia that goes around the fan at the top. Pull the top of it down and it unclips.
You will then see two screws either side of the fan which can now be removed.
Looking further towards the bottom on either side, the are two more screw holes. Undo these. There is a design fault in that these screws go into exposed plastic lugs. During icing up, it brock the lugs away from the back of the fridge to I can no longer tighten the screws. Do not be surprised it yours are the same and you cannot actually unscrew the screws. They will just keep going round as they are not attached to anything.
Carefully pull the whole back section containing fan towards you. Take care if lots of ice is present, you do not want to pull the cooling fins away as well.
You need to twist the back section so the left side comes out first (it is tight), but it will allow you to reach round and un plug the red wire which powers the fan. Don't worry about looking at the plug, it only goes in to the top connector one way round.
Now remove the whole back and fan housing. Be careful of at polystyrene panel on the back of this housing.
Now you can carefully use a hairdryer to melt all the ice around the fins. There is another stainless steel shroud that goes over the fins, This simple slips off as well if you wish.
Once the ice has melted, re install and it will work fine again.
Melting the ice like this takes about 3-4 hours rather than 24-48 hours.