Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
Please confirm the make and model number of the box?
On the QAS-1545, if you don't know the code then the only way in is with the key.
If you lose your key, you will need to obtain a replacement key by referencing the serial number located on the front of the door.
You can request a replacement key by calling Stack-On Customer Services on 1-***-***-**** (M-F, 8-4:30 CST)
or filling in the following form: https://www.stack-on.com/downloads/1
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There isn't a default code, only the key can be used however you can get a replacement key from Stack-On.
If it is a dire emergency then you could have a local locksmith or handyman force it open for you;
otherwise your next best option is to call Stack-On and have them express deliver a replacement key to you.