I have Samsung RSA1RTMG fridge freezer?

The Temperature light on the freezer was flashing this morning on -21, i then have unplugged the fridge freezer and plugged it back in.. where the temperature has went to 3.. i have tried changing it back to -21 but it keeps jumping back to 3.. the food in the freezer has started to defrost, I have put on 'power freeze' for the time being, i just need to be able to save the temperature at -21.. Thanks