If the chair is wood use an aliphatic resin (Titebond or Elmer's Wood glue which you can get at Home Depot) and clamp the repair in place for 24 hours. If the dowel end of the chair leg does not fit snuggly in the hole any more you'll have to fill it with a glued-in dowel and rebore it. This is the way I've always reattached wooden chair legs and it's always worked.