If I am being sued for a medical bill thatI never received because they had the wrong address, what canI do?

The real issue is, is the bill--now that you've received it--valid; that is, do you really owe the money? If you do, you should probably try to pay it in exchange for the dismissal of the lawsuit and a release from further liability; in your offer, explain that  you never received the bill. Make an offer in writing, and send it some way that you can prove that you sent it (e.g. certified mail with return receipt; fed ex; fax and keep the confirmation sheet; etc.). If the creditor or collections agency insists on getting some additional interests or fees, then you have to decide whether it is worth paying that additional amount, or whether you'd prefer to fight it, incurring the cost and time of litigation, in order to tryy to show in court that the only reason for your failure to pay is that you did not receive the bill initially, because it was sent to the wrong address.

If the bill is valid, you'll have to pay it (except as set out below); therefore, assume you'll owe that much money. You should not owe additional amounts, however, if not paying was not your fault--though sometimes, it's worth paying the extra, to avoid litigation.

Note that every state has what's called a "statute of limitations" for debts--for this case, it's probably the statute of limitations, or SOL, that would apply to written contracts. You don't indicate what your state is; you should either repost the question with the state, or else look up the SOL yourself, since IF the debt is  older than the SOL, it may be too late for them to enforce it at all.