The weather has been exceptionally cold for a long time ( I have a degree in stating the bleedin' obvious ! ) so chances are that unless you had at least 50/50 mix of water and washer fluid then it will have frozen.
Before the weather gets bad again fill the bottle with that mix. Make sure that the front washers work first time, every time and then try the rear washer. Obviously you can make sure the jets are clear with a pin but I assume they were working ok before the big freeze.?
If it doesn't work then you need to check in the boot that no water is appearing unexpectedly because if it is then it's likely that the pipe has popped off .
Only way to sort that is to remove the boot lid card and replace the pipe.
It's a common fault on A3's apparently although, touch wood, I have been lucky so far !!!