Sorry, I thought you meant a communal or shared box.
In which case if it's an individual box's lock, occasionally there's a number on the lock or original key. That number corresponds to a pattern which the manufacturer or some key cutter or locksmith outlets can use to make a duplicate key. Alternatively, a locksmith (and some key cutting shops if they have locksmith experience) should be able to open the lock in about 5 seconds but can also make a key if you show ID. In all cases, ID may be asked for, for obvious reasons! (There's also a chance the building admin or managers have a spare - so ask them first before going down the locksmith route.)