I have Ubuntu 12.04 installed like an app on windows 8.1 (Ubuntu 12.04 allow to be installed like an app in Windows 8.1 and it can be removed when you don't need it any more from Control Panel). Usually, to chose what os to boot when you start the laptop, you can choose between windows 8.1 and Ubuntu after windows logo appeared at start up and that was ok until I made this upgrade. Now when I try to choose Ubuntu the laptop try to boot it but, after that full colored screen is showed the screen go black and this messages appear:

mount: mounting /dev/loop0/ on /root failed : Invalid argument
mount: mounting /dev on /root/dev failed: No such file or directory
mount: mounting /sys on /root/sys failed: No such file or directory
mount: mounting /proc on /root/proc failed: No such file or directory
Target filesystem doesn' t have requested /sbin/init
No init found. Try passing init = bootarg.

BusyBox v1.21.1 (Ubuntu 1:1:21.0-1ubuntu1) built-in shell (ash)
Enter 'help' for a list of built-in commands

(initramfs) _

I don' t know what to do after this screen appears. Please help !