If the little plastic 'scissors' is still in place, and all you have is the key top...
Place the key top in position, and press firmly down with kind of a rolling motion across the key, and that should pop it back in place. That's the only way I can describe it. These keys are easily removed by popping the key up from the left side, so you could apply pressure on the Right side first. They're tricky little things sometimes.
If the scissors is also off - I have told people sometimes to pop another key off, and take a look at that one, but you run a risk of having 2 keys that you can't put back.