Sometimes your character can get stuck or venture into a region that doesn't allow you to interact with your minecraft server or even connect to it anymore.

One way to resolve this is to delete your player.dat file. Just be warned this will delete your inventory so this is to be done as a last resort or if you don't care about your inventory.

To delete the file go to: minecraft/world/players then find your character name and delete the .dat file there.

Next restart your server and your player will be reset and back at your spawn point again.