Examine the remaining crystals on the shirt, taking note of the size, shape and color of the stones.
Find flat-back crystals to match the ones on the shirt at a craft store. You may find it helpful to bring the shirt with you so that you can compare. Pay attention to the details of the crystal, including its reflectivity and facets. The more closely you're able to match the rhinestone, the less obvious your repair will be.
Lay the shirt out on a flat work surface and place a layer of newspaper between the layers of fabric so that the adhesive will not bleed through.
Carefully mark the spot where the missing crystal should go using fabric chalk. Place a new crystal on the chalk mark and check that the placement appears correct both up close and from far away. Adjust the mark if necessary.
Pick up the replacement crystal and apply a small amount of fabric adhesive to the back; press it firmly onto your chalk mark.
Allow the adhesive to dry fully according to the instructions on the package.
Wash the shirt to remove any excess fabric chalk marks, then wear as you normally would.