What machine do you have? If it is anything but Facetron, then take the dop out of the key so that it rotates freely. Then lower the facting head till the quill is parallel to your flattest lap or at the angle 90 degrees. Rest the stone on the lap and rotate it till you fill a flat spot. Then while holding the stone down on the flat spot, tighten it in the quill. You should be ready to go. But understand, that you only lined up the girdle facet in relation to the index position. It may still be slightly off based on its original position on the dop. so you will have to recut all of the facets you were working on before the stone came off.
Another thing to consider, is if you were using dop wax as your binding agent, when the stone came off there is a pretty clear impression of where the bond broke and with a little patience and a steady hand you can usually get it even closer to perfect. But you still need to recut everything you were working on.