I have Ubuntu 14.04 installed in an external hard disk for testing. Last night I was ripping a dvd with handbrake and fell asleep. This morning when I woke up I saw the ubuntu login screen (unity greeter) I entered my password repeatedly but unable to login. So I shut down the machine, reboot it and removed the dvd then tried to boot off the external drive, still no luck. I saw black screen, no grub, no nothing.
So I removed the external hard drive, booted into Ubuntu 13.10 which is my working system and is installed in the internal hard drive. Then attached the external drive and viewed it in gparted. It complained that the /home partition of 14.04 has "unknown file system' with a big black or red mark. So apparently somehow I toasted the /home partition with my dvd burning.
I then reformatted the /home and restored it with an image but still wouldn't boot. I then booted into 13.10 again, mounted the 14.04 / partition and reinstall grub and now it works.
I am wondering what could have happened?