Fixed in 188.8.131.52 / 16.03.2016
If you missing HUD and client restart doesn't help you.. and you do not remove all elements by your own with F10 or Ctrl+H :)
http://steamcommunity.com/app/290080/discussions/4/521643320357118707/ and check solution provided for this bug.
There is also one more way to fix this issue and you might like it more
Go to: SteamLibrary\steamapps\common\Life is Feudal Your Own\data\presets\HUD and delete everything in there. Next time you start the game your HUD/UI will be back.
1. Game menu
3. Scroll to bottom and press Reset next to HUD presets menu
This is known bug and our team already searching for root problem in order to eliminate it, but at this time please use workaround
bug 4533 (registration on lifeisfeudal.com required)