You've lost the key to your mailbox -- but you don't have to lose hope, too. There are ways to obtain a replacement key. And if that fails, you can have the lock changed. While you pursue a solution, don't forget that your mail may have time-sensitive items. Taking care of a lost mailbox key also involves taking care of the mail.
Retrieving Another Key
If you live in a rental property, start by checking with your landlord. Apartment complexes often keep copies of mailbox keys for just such an emergency. If the lost key opened a post-office box, then inform the clerks at the post office. They should have a replacement, although they will probably charge you for a new key. You will need to prove your identity.
Replacing the Lock
If you can't get a hold of a new key, you can get a new lock. A locksmith can drill out the lock for you. Examine the mailbox for the name of the manufacturer. If you can't find one, look on the lock itself. The mailbox or lock maker may be able to tell you what kind of replacement lock you need.
Don't Forget the Mail
While you figure out how to get into the mailbox, be aware that bills and other important material may be piling up inside. Contact the companies and services that issue your regular bills and explain the situation. They should be able to arrange for you to pay online or by phone.