Rebooting didn't work and Amazon have said they won't replace it, though they have offered me a discount off a new one. I'm pretty irritated though. 18 months is a crap lifespan. It was kept in a case, loved and adored. Never dropped, got wet etc. It should have survived longer than that.
Can you check you have the latest updates installed? My 3yr old keyboard one died last week, and I rang up and they said I hadn't got the updates, and when they were installed t came back to life! Worth a try..