My post and crown recently fell out. My question is what's left in my gum?

This is likely the root. Sounds like the post broke off with the crown attached to it. Make an appointment with a local dentist to see what your best treatment options are. 

Hi:What is left is the root of the tooth.  If there is no damage to the tooth and the post and crown fits properly, you can have it simply recemented.Best,Dr. Maddahi

what’s left is the root of your tooth. That probably needs to extracted.  From there you have 3 basic options. An implant which acts the most like a tooth. A bridge which uses the tooth on each side to hold in a fake tooth (3 crowns fused together is how I try to explain a bridge sometimes) or your last option is a removable appliance. Hope this helps

Joe

A post is used in a tooth that has had a root canal and has very little foundation left to support a crown.  When the post and crown comes out, it may be recemented back in if you see your dentist quickly.  However, the best dentistry may be an implant.  The remaining tooth may no longer be able to support the post-crown, which is probably why it fell out in the first place.  Good luck!