Depends mostly how is attached. Most knifes are built like swords were, and in fact they are small swords, so there's an hilt and a tang/shank that usually fits in the hilt/handle.
I don't know what's exactly your knife, but check if there's still a recognizable shank and the handle isn't too much damaged. If there is, you could find an atoxic adhesive and simply fix it.
I've heard Sugru is quite strong, and safe for stuff meant to be in contact with food, but since a shank allows for a very little space, you may prefer some good-quality epoxy.
If the shank is broken too... well, that's much like trying to fix a broken blade. And a broken blade with a Mohs Hardness number just a notch behind diamonds (8,5, where diamonds are a solid 10).
And that, as fair as I know, makes repair nigh impossible.
If there's still a shank, but the hilt is completely worn out, you may:
1. If the handle is riveted like in the generic picture, and the rivets/fastener
2. If the handle is somewhat usable, but the groove in the hilt is too damaged or enlarged to allow the shank to ever fit again, you may simply use epoxy or Sugru to "fill it", restoring a correct grip and adhesive power
3. If the handle is generic plastic/fake wood and damaged beyond repair, you could get creative and use Sugru and similar materials to make a new one around the shank. Also, you could use wood and epoxy, getting really creative and, if you've good hands, getting something that looks still snazzy and professional.
If that doesn't cover the type of damage you've got, please, give us more details about how the blade got disconnected, and how blade and handle look now.
I mentioned Sugru and Epoxy because I'm sure of their atoxicity: despite having heard people using Gorilla Glue and other fast acting glues, I would keep them for Renaissance Fairs props... not for something destined to make contact with the food you're going to eat or give to people you like.