The ubuntu-desktop in synaptic is removed on my system. And when I start the machine, I cannot login ,the mouse and keyboard cannot be used also. How to fix it? How to enter the text mode of ubuntu?
Select the recovery mode in the grub screen (if it doesn't show up, keep pressed the shift key during boot). Then if you can login as root, try to install again ubuntu-desktop (this may install missing packages). If only "ubuntu-desktop" package was removed, then it should be other problem, as it is a meta-package which is not essential (can be removed safety).