While booting, I am getting the error message internal error: the second sector of Stage 2 is unknown." This is under Ubuntu 10.04 Lucid Lynx. After posting, the full text of the GRUB loading screen is:
GRUB Loading stage1.5
GRUB loading, please wait....
internal error: the second sector of Stage 2 is unknown.
This is an existing install that has been working for a long time. I came home to find the computer powered but not responding with no input to the monitor. Upon rebooting, this error message was displayed.
Best Answer
I would try reinstalling GRUB2 from the Live CD; if this errors out, this should give you at least some hints about what is going wrong with the system. (The fact that you found the computer non-functional after leaving it powered on for some time suggests some hardware problem happened.)
There are step by step tutorials on the Ubuntu wiki and on FOSSwire (more command-line oriented).