Condo or apartment mailbox lock without key?
What you can do - Meet your mail delivery person when the whole box is opened to insert the mail. Have a flashlight, flat screwdriver and pliers ready to reach into your box. Detach your lock from the inside. Most are held in place by a clip or nut. Once you get the clip or nut off, you may have to bend the cam on the back of the lock to slide the lock out. Do not pry or damage the door or frame. Take the lock to a locksmith or hardware store. There are many different locks available (and a few that are no longer available).
What a locksmith can do - Since the box will be locked when the locksmith arrives, he will first open the lock. Then he will replace your lock with a new one.
If you need a duplicate made of your mailbox key, bring your key to your local locksmith shop.