To get your character unstuck, there are a couple things you can do.
If on Official Servers, you would need to do the following:
If your character is stuck in mid-air you can punch until your character is fatigued and continue punching which will in turn drain your character's food when attempting to refill your character's stamina thus starving your character.
If in Single-player, you are actually a host and admin of your server, Just hit your console key, defaulted to TAB on your keyboard and type in the following string:
After doing so, you can type the following command to kill yourself
cheat hurtme VALUE
with the Value being a whole-number higher than your current HP
Please note that this does not include the Z axis, so you may fall from high ground, or appear lower than intended. To get above ground, you can just increase the 3rd value on the console command, and to be safe from falling from a high-distance you can enable the console command “cheat god” however this will put your character in an unkillable state, so remember to turn it off afterwards, by activating it again if you would like to continue playing as normal.
Please submit a ticket if none of the steps above fixed your issue.