Bluetooth keyboard stopped working after upgrading to Yosemite
I seem  to have fixed this now.  


I went into Bluetooth prefs and removed the mouse and Keyboard.
I removed to batteries from both.
Rebooted.
Went to System Preferences → Mouse
Put batteries back into mouse, re-paired mouse only
Rebooted.
Went to System Preferences → Keyboard → Connect Bluetooth Keyboard.
Re-paired keyboard.