I have just had the same issue as you, first thing in a morning, restart after 5 minutes and it would all be OK again - and my horn also stopped working at the same time. New battery solved it.
After reading some other people's issue with the horn not working, and suggestions that it is related to the battery, I recalled my a comment from my service last November that the battery had 'failed' the dealer checks. I thought nothing more of it since the car was starting fine, even on frosty mornings. Last week I charged my battery over night to test the horn theory, and for the next 3/4 days I had no issues. Bought a new battery and fitted Saturday and so far it's not skipped a beat!