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Safe batteries going dead tend to be a royal pain to deal with.
In the case of some Sentry safes, there is a keylock on the safe also. (It might be under a protective sliding hatch or cover, to keep dust and dirt out of the works.) That is for the emergency key, which can open the safe in a case like this, when the combination has been lost or forgotten, or the batteries have died.
Do you see any keylock on the safe, or know of any key for it?
If so, open the safe that way for now. It's possible that the batteries for the electronic keypad need to be replaced, esp. after 4 years use, and esp. if it shows no signs of life, no 'beep' in response, no lights, etc.
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