It is not uncommon for the lock to expire prior to closing, this can happen if the escrow has been extended (which is most likely the case) or if an interest rate has been locked-in prior to receiving a fully accepted offer. I would suggest you contact your lender to find out what their rules are for extending rate-locks.

Extending locks is common practice...if the rates are better today then when you locked in then there should be no fee for extending, or a very minimal fee. However, if the rates are worse now than when you locked in your rate then you may have to pay for the extension.

Most major banks allow for extensions...in fact there should really be no reason that you could not extend your rate lock.