Can the dentist reuse your old gold crown on the same tooth after a root canal?

depends on the reason the crown had to be taken off
if it was for caries the crown should be replaced
if it happened just because the pulp started hurting but with no caries involved he might be able to glue the crown with temporary glue preform the root canal and then glue it again permanently -though he will have to add a `core` to the tooth too.