update 2016/07/01 6pm: SOLVED. Details here (but DO read anyway the whole post, because the cause of this problem on your computer may still be one of those below!) I just installed Ubuntu 16.04 LTS on a relative’s laptop (HP Pavilion G6, in case it matters) alongside Windows 8. So far, everything works so well (*) that Ubuntu doesn’t want to shut down. If I click on shutdown, the user session ends, and nothing happens.