I just changed the battery in my HP 48GX that I

Hello,
The following procedure valid for the HP 48G should work for the HP48GX.
Make the calculator perform a
Self test.

1. Turn calculator on
2. Press and HOLD the key [ON]
3. Press and release the key with "E" to its right (the one before last in first row)
4. Relaese the [ON] key.

It tests the RAM and ROM. You will see various patterns on screen. Afterwards you should see
IROM OK,
IRAM OK,
HP48GX OK
The self test will continue repeatedly until you perform a
System Halt

1. Press and HOLD the [ON] key
2. Press and release the "C" key
3. Release the [ON] key.

The empty stack should appear.

If you do not see IROM OK and IRAM OK, the calculator requires service.

If the above procedure is successful, you may want
to test the keyboard.

1. Turn calculator on
2. Press and HOLD the [ON] key
3. Press and release the "D" key.
4. Release the [ON] key
5. Press and release the "E" key. KBD 1 should appear on the top left corner of the screen.
6. Starting at the upper left corner of the keyboard, and moving Left to right, press every key in sequence. If you follow the sequence and the keys are functioning properly you should hear a hight pitch sound at each press.
   
7. If everything is OK, when you press the last key (+) the screen displays KBD 1 OK.

To exit the keyboard test proceed as described above under System Halt.

If you follow the proper sequence and a hexadecimal number appears after pressing a key, the calculator requires service.
Hope it helps.