After changing the battery in the key it no longer locks/unlocks the car or disarms the imobilizer. My second key still works.

My car is a 2004 Legacy. The key is a Subaru branded key with one button and has an oblong head. It is not like the two keys shown in the handbook which shows a 2 button key and a taper headed key.

Does the key need reprogramming? If so can I do this myself or do I need to go to a dealer?

The handbook also mentions a keypad but I have not got one.