faulty freezer

help please i own a hotpoint fa90 model fridge freezer which has been working fine for the last 4 years. a few days ago it started to play up i get a warning message on the digital display that the temp is 0 then minus 1 and so on. i turned it off and back on and used the fast freeze button to see what would happen , it was okay for awhile but the same thing is happening again, i need to fault find myself if i can to see what the problem is also i have noticed ice build up in the bottom of the freezer, it is a frost free freezer, also the coils at the back seem preety cold or is this normal . i want to try and fix myself so please helpppp 🙁 🙁 🙁