Q: No longer able to print from iphone 4s to hp officejet pro 8600 plus using airprint

Double click the home button and see if the Print app is open. If it is press and hold it, tap the dashed line to close it then tap the home button. Next reset your iPhone, Router and printer.

Read http://www.apple.com/support/iphone/assistant/airprint/

Similar problem and I was going crazy, as AirPrint and Bonjour printing suddenly stopped working out of the blue.

After two days of restarts and reconfigs, I finally noticed that the date/time on the HP 8600 was wrong, specifically 20 minutes behind.  I tapped on the time on the panel of the 8600, updated it to the current time, and cleared the print queues on my devices.  Everything then came back to life! 

I guess Bonjour (which is also used by AirPrint) requires the clocks on the networked devices to be somewhat syncrhonized for everything to work.