Grub is not loading ubuntu12.04

Hi All,

First of all thanks to all sharing all kind of useful info.
I have one similar problem, want to take inputs from you.
In my Laptop I am using wind7 and Ubuntu12.04 as dual boot.
it worked fine many days, recently I am seeing one problem.
Grub is showing both Wind7 and Ubuntu in boot options, but when I select Ubuntu it is loading only black screen with white cursor blinking. If I select wind7 it is loading wind correctly and working fine. Only problem with Ubuntu loading.
It worked previously, now only it is giving problem. What way I have to proceed. I need inputs from you.
As I have important data in Ubuntu partition I don’t want to experiment my own loosing data.
Thanks in advance.