Linux: How to Restart X11

switch to a virtual terminal, by pressing 【Ctrl+Alt+F1】 〔►see Linux: How to use Virtual Console〕

(if even that key doesn't work, try Alt+sysrq+r )

then

sudo service lightdm restart

then

press 【Ctrl+Alt+F7】 to go back to X11. 〔►see Linux: How to use Virtual Console〕