Originally Posted by AmandaK
Thaw it. Gravity reading. Taste it.
If it needs to attenuate more, repitch a huge starter at high kreusen. Then proceed as usual.
I'd be more inclined to just pitch yeast instead of yeast plus 5 litres of oxidized wort.
Pitch rehydrated dry yeast if you think you must repitch, or yes, go through the entire process of liquid yeast, a starter, chill, decant, and pitch.
Your yeast may very well be okay anyway. They are tough little bastards. Like someone else said, take a gravity reading and see where you are at.