Apple keyboard will not pair

I had no problem pairing my macbook pro with my Apple Wireless keyboard for a couple of years. It has been a week or two since I last used it. Now it will not pair.

I changed the batteries. Pushing the keyboard power does bring the light on the keyboard, but it says solid for a few seconds and then goes off. It does not start flashing, which the manual says it will do if it is in discovery mode.

Any ideas on what might be going wrong?

Thanks

Rob

P.S. I am running Lion 10.7.3 if that matters.