The only way to open it is with override key or the factory override code, if your user code is not known.
If you cannot find it, you can order a replacement from Brinks, or really from First Alert/Honeywell (the parent company)
In order to claim the replacement, you will need to write them a letter including the key, model, and serial number for your safe, as well as contact information and your address. You will need to get this letter notarized by a registered notary public. Once you do, send the letter with a signed check or money order for the amount listed and they'll mail back a key.
You can get the form HERE
I am an independent technician and I've spent my personal time, unpaid, in a good faith effort to assist with the question for which you solicited my services. I am only paid for my efforts when customers such as yourself leave positive feedback for my work. Again I apologize there is not a better outcome here, though this is really beyond either of our control. I do hope you’ll consider leaving a positive rating for me, rather than “shoot the messenger”, so that I may get credit for my time spent in good faith to answer your question accurately.