If that is a wooden post I it is probably rotted off. And I expect it was
set in concrete. The tough job is getting the old concrete out. I prefer
to tamp the dirt in around the post and not use concrete. That is a little
more work when installing but a lot easier when you have to remove them
again.

If it is a metal post you probably will have to set it in concrete.

OP here. Thanks for everyone's suggestions.

It's a 6 ft fence. Standard fence that builders put. The posts are
wooden. Not metal. I don't think it's rotten because it's about 4 years
old and it looks in good shape. My backyard has a retaining wall made
of railroad ties that's supporting the dirt. So I figure the post just
became loose or something.

Thanks.