It seems to be a glitch or bug. Its been officially acknowledged and addressed by Apple. There is no resolution other than waiting for a few days. The Kb article by Apple states it will fix itself on January 7.
After January 1st, 2013, Do Not Disturb mode stays on past its scheduled end time.
Do Not Disturb scheduling feature will resume normal functionality after January 7, 2013. Before this date, you should manually turn the Do Not Disturb feature on or off.
To turn off the scheduling feature, tap Settings > Notifications > Do Not Disturb and switch Scheduled to Off.