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The number pad won't light up until a button is pressed (even if power is applied), as far as most models of Sentry safe are concerned.
Did the old batteries leak at all, or look corroded or browned on the ends or terminals? if so, you need to clean the terminals in the battery compartment with cotton moistened with alcohol, until they are shiny again, then allow at least 5 minutes for drying off time, then try batteries again.
Does your safe have a key lock? if so, use the emergency key on it, to open the safe. That is what emergency keys are for, to help open a safe when it has an electronic keypad and the batteries have failed.
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