Typically, once your rate expires, you are out of luck on that end of things. If it is the fault of the seller, you may be able to negotiate some form of credit back to you, but I know that in Virginia, that is not part of our standard sales contract, so would require some additional language to be added and then agreed upon.
Has your rate expired already? If not, why not discuss with the lender buying an extension on the rate lock? Once you know the costs involved, talk to your Realtor to see if there is any likelihood that you will be able to get help with them or reimbursement from the seller. It's worth a try, although probably not very likely. How long have you had a ratified contract?