Whirlpool a class freezer

Question

i have a whirlpool a class freezer that fits under my kitchen unit. It has defrosted and I dont know why??Can anyone help??There are three lights, a yellow one that is on, a red flashing light (it doesnt usually flash)and a green light, therefore the electricity is on but it isnt freezing...any advice?? I have lost my manual and dont know what I need to do to fix it.Thanks