If the post that is inside the crown and down the root of the tooth is fractured it needs to be carefully removed from the canal. If the tooth structure is sound and there is enough left to support a new crown then a new post and crown would be indicated. A lot of times if the post fractured it indicates additional problems:1. There is insufficient tooth providing structural support to the crown or2. There was fracture of the tooth that was supporting the crown/postYour dentist needs to x-ray the tooth as well as evaluation of the surrounding bone/gum tissue to determine if there is enough solid tooth remaining to warrant restoration with a new post/core and crown. If not, look into having the tooth removed and restoring with an implant or bridge.
Typically when a post and crown come out. A new post and crown is needed. Trying to retrofit an old crown to a new post is a serious compromise and may leed to future problems.