I believe the same keypad is used on the SB0500 and SB3510 model.
@OP have you changed out the batteries yet? Are the battery terminals clean or corroded? Can you enter 5 5 5 5 5 and see if anything beeps or lights up?
You can always call a local locksmith and they can call Sentry, provide them the serial number, and hopefully Sentry would be able to look up the combination, but it's such an old model, who knows if they even have that info. That call the locksmith makes should not cost you much, but the locksmith coming to your house to verify ownership of the safe may cost you a service call fee for them to drive out.
You can also try this SentrySafe online form:
https://replacements.masterlock.com/products/lost-combinationSquelchtone
Here are the instructions for your safe:
large version click here:
https://i.imgur.com/aeU3FFy.png