Opening a trunk without a key is a relatively easy task if you do not care about the condition of the trunk. If you want to preserve the trunk and the locking mechanism, exercise care in unlocking the lock or gaining access to the inside.
Pick the Lock
If you don't throw out old keys, go through your collection or borrow some keys to see if one does the job. If that doesn't work, use a pair of bent-nosed pliers or a metal coat hanger to turn the tumbling mechanism inside the lock. Old trunks have fairly simple locking devices because they were designed to keep the trunk lid shut rather than to prevent burglary.
Remove the Hinges
Remove the hinges holding the lid onto the back of the trunk body. If you can't unscrew them, use a cut-off saw or angle grinder to sever the rivets holding the hinges. In difficult cases, drill out the rivets. Many antique trunks have significant monetary value, and if you want to maintain your trunk for restoration, contact a professional restorer, who will also open your trunk.
Use a Locksmith
Any competent professional locksmith is able to open a lock on a trunk. To be assured of quality service and fair trading, employ a member of the Associated Locksmiths of America, Inc. (ALOA). Members of the ALOA ascribe to a standard code of ethics that includes licensing, non-deceptive practices and presentation of "the best possible solution" for the client.
Depending on the age and quality of your trunk, what you are opening may be valuable, and what is inside may be more valuable if you do not know what it contains. It is important to know whether you have a hide trunk, a dome top, a round top, slatted or not, a Jenny Lind, monitor top or flat top. Know the type of trunk you are opening before contacting a restorer.
If All Else Fails
Assuming the value of the trunk is irrelevant and you have exhausted all attempts to open the lock, dismantle the boot. Cut, saw, hack, hammer, pry or by other means remove the locking mechanism or hinges--whichever seems simpler to dislodge. Cut out the bottom if need be. Be prepared to cut out the bottom of any tray you find inside that may block access to the lock or hinges. You will need to retrofit or replace the bottom as a result, but you will have succeeded in opening the antique boot without a key.