To follow up on Tha Riddla's answer:
If your freewheel is freewheeling in both directions, in all likelihood it is gummed up inside and the pawls are stuck open. (As a small note: the freewheel is separate from the hub and contains bearings, pawls, springs, and some lubricant.) You can either try to overhaul the freewheel or purchase a new one (they range from relatively inexpensive to bank-breaking). To replace the freewheel, you can either buy a freewheel remover or take it to a shop and ask them to replace it (the whole procedure should take 15 minutes).
Some freewheels are difficult to crack open, so the simplest (and cost effective) overhaul would be to drip a medium-weight oil onto the seam where the cogs and freewheel body meet. Be sure to thoroughly wipe off the area with a clean rag before you start. You can do this WITHOUT removing the freewheel and you should hear a difference immediately. Put the wheel over a rag and continue dripping oil and spinning the freewheel until clean oil comes out of the back side of the freewheel. This should loosen up the pawls and clear out the gunk inside.