How can I fix my Hotpoint Mistral Plus Fridge Freezer?

I noticed the other day my freezer temperature had risen to -5 so I put it on fast freeze as this usually fixes the problem straight away. The next day I noticed it wasn't even making a noise(its usually loud) and that the temp had risen again to -3 so I pulled it out to make sure the air could get behind it and it instantly dropped to -8 so I thought it was fixed, couple hours later it made a loud bang and it had risen to -5. I then noticed it goes from -3 to -8 then seems to just stop working. Any ideas on how to fix this so it can get back to normal temperature?