I have a gun safe... I went to change the password/code, but forgot to open the door while doing so... now none of the codes work including the one issued when purchased. I am wondering if it is the battery?

Does it not have a Key over-ride? To allow you to lock it and unlock it

If it takes a Key as well:
| Sentry® Safe | Need a Key (http://www.sentrysafe.com/CustomerCare/Need_A_Key)