Grey, every fall, I take all my geraniums (about 10 -12 now), knock the dirt off them, and pot them in a large pot with some dirt, but it ends up being mostly roots. I move the pot indoors near a window. I water them about once a month, and they get pretty messy and spindly looking by spring. When spring comes, I take them all out of the big pot, repot them into their own pots, prune them right back, water, and watch them grow. It's actually amazing - I think you could measure how much they grew every day.
As long as the roots didn't get frozen through, it will be fine. It actually does them good to get cut back once in a while.