Happened twice now. Have gone in for a landing, messed it up, went into Go Around, set a manual heading, and after a bit went in to SEC F-PLN and clicked ACTIVATE SEC. At that point all systems stop responding. I can turn dials on the FCU, which rotate visually, but the numbers don't change. Same with other things in the cockpit. I can click, and the animation of the click shows, but nothing happens. Only thing that I could actually do that had an impact was to retract the flaps, which had aerodynamic effects, but did not lower them visually on the airplane and did not show any changes on the displays.