Are you saying you bottled the beer, or is it sitting in a carboy or bucket? If your beer is bottled, you are pretty much SOL and are stuck with bottled beer.
If its in a carboy you can transfer to the keg and do 1 of 2 things:
1 - purge with CO2, prime with sugar and let it sit for a few weeks. This way, it will carbonate while its aging. There are a lot of threads here on how to use sugar in a keg. You will end up using less than if you bottled, but I can't tell you the exact amount
2 - rack to keg, purge with CO2, let it sit at room temp for aging. When done aging, put it in the kegerator, keep it hooked up to CO2 on ~12psi for a week or two until its carbonated.