It's a dangerous time for fledglings and there are a few possibilities as to what's happened.
From the time the young are old enough to squawk for food they give away the position of the nest, so become vunerable to Magpies and other predators, and of course when they make their first tentative solo flights from the nest they're at risk from predators again. The mortality rate is high.
It's not unusual either for the young to either fall from the nest or be unable to get back from their first flight. It's not all bad news though, these young will often take cover and the parents will feed them where they are. A good percentage of these young will probably survive.
All you can really do is let nature take it's course.