I don't know if changing the battery will affect the code or not. I can make some guess based on the model of the opener, but not from the keypad.

But even then it is only a guess.

But most battery operated devices will retain their settings for a few minutes while you change batteries. But that is no quarantee that it will in this case.

And realize that there are many of possibilities of that could cause it. Failure of the opener. Failure of the door that overloads the opener. Failure of the electric supply.

I would check the breakers to make sure that they are all on. And also check for an GFCI receptacles that might feed the garage. And they can be in strange places. Depending on the age of the house it could even be in one of the bathrooms or the basent.

Past that I don't have any ideas. Is there a window in the garage? What is the siding? If it is vynil it would not be to hard to removing and cut through the sheathing to get in.

But no mater what is the problem and fix YOU NEED TO INSTALL A REMOTE OPENER RELEASE.

Here is one.

http://www.chamberlain-diy.com/diy04/products/buy_online_door_control_models/0,1341,uid-d3VmbWFiYGdpZg==_1207,00.html

Chamberlain makes Sears Openers so it might be available from Sears, but if so it would have a Sears name and number.

Here is the Liftmaster version (also makde by Chamberlain).

http://www.aaaremotes.com/lifg1.html

Here is a universal version.

http://www.aaaremotes.com/key-release-lock.html

After you get into the garage you can look up the opener model here.

http://www.aaaremotes.com/liftmastercompatibilitychart.html

Then click on the Remote column and you will find the remote for this unit.

Also on that page you will find the programing information.

And also on the page it shows the compatible keypads. Going there you will find the programing information for the keypads.