Alarm going off to consume all food in the freezer

Question

Our Hotpoint FF45 has been bleeping and giving out the message to consume all food in the freezer. We have defrosted it by emptying it and leaving the door open overnight. But when we switched it on again the same message appeared.
I looked round the back and there was a black water tank underneath about half full of water. The round black ball thing felt quite hot too. We have had some building work done and I wonder if dust might have got in somewhere.
Is there anything we could do to make it work properly again?
Thanks.
Wendy