If your ubuntu gnome crashes while you working on it,and nothing you have entered seems to work.Following steps may help you reboot the frozen ubuntu in the safe way.It’s far better than pushing that power button.
Try these steps:
Hold down Alt+Sys Rq(PrtScn key)key combination.
There will be no visible output,while hold down the key combination,
Then press these keys in the order,one by one
Spells “busier” backwards,and the system will be reboot after B is typed in the safe way.