I've already read about this problem and its reason was a fault taken by Blizzard concerning the servers. Best would be if you directly contact Blizzard about this.
In the two cases I read about this, the characters were gone.
Edit: This is the appropriate link to contact Blizzard about this:
http://us.blizzard.com/support/diablowebform-us.xmlThis post was edited by iDGames on Jun 12 2011 03:33pm