原因:可能是图形桌面崩了;
可ALT+CLT+F1/F2/F3/F4/F5/F6,进入终端界面,重装桌面:
sudo apt-get update
sudo apt-get autoremove lightdm
sudo apt-get autoremove unity
sudo apt-get autoremove ubuntu-desktop
sudo apt-get install lightdm
sudo apt-get install unity
sudo apt-get install ubuntu-desktop
sudo service lightdm(或者gdm3) start