Something odd happened to my iPhone

All the notes in the Notes app made after 11 May 2012 and before this morning have vanished from my phone. I can still make new notes, the old notes are still there in the Notes app on the Mac and in the Notes app on the iPad, though notes made on the phone don't seem to be syncing to the Mac.

Quitting the Notes app and restarting it doesn't help. The power switch on the phone has broken, so I can't try rebooting the phone.

Any advice?

Conclusion I got the power switch repaired (at the shop at the Milton Road / Arbury Road junction); disabling and then re-enabling synchronisation with iCloud made the notes come back. My contacts had also vanished, but again disabling and re-enabling synchronisation brought them back.

I suspect 11 May 2012 is relevant because it'll have been the day I updated the phone to iOS 5.1.2