When a diamond or gemstone falls out, replacing the stone is only part of the job. The more important first question is why it became loose.
Keep the jewellery safe
Stop wearing the piece, keep any recovered stone separately in a secure container, and avoid trying to push it back into the setting yourself.
Assess the setting first
Worn claws, impact damage, distortion or an earlier repair can all contribute to stone loss. Repairing the setting before selecting and fitting a replacement helps reduce the chance of the same problem recurring.