Did you know the iPhone has a print queue? When you print to an air print ready printer the iPhone, like a computer, stores the data in the print queue until the wireless printer accepts the data and prints it. If your iPhone stops printing, you may have some data “stuck” in the queue.
To empty the queue and resume printing from the iPhone, double tap, (two quick taps), on the home button. The home button is the same button you push to begin using the apps. When you double tap the home button, you then slide to the right and look for the Print Center. Select the print jobs and cancel each one. You should now be able to print. The print center only appears if there are items in the print queue.
If that fails, then power cycle the printer and iPhone to re-establish the wireless connection. One other note: make sure you are on the same network.