In fact, in addition to fault mbr boot sector appears unable to enter the operating system will cause the outside, gurb menu failure will not enter the operating system.
GRUB reasons for failure
(1) GRUB boot loader in the MBR sector suffered accidental damage
(2) grub.conf boot configuration file is missing or incorrect
this kind of failure manifestations
system boot stagnation, it has been stuck in. " grub> "symbol in
fix ideas
(1) the boot CD into emergency mode, overwrite (that most people do not technology) from a backup or restore grub.conf
(2) reconstruction procedure to the grub MBR sector
specific repair step
(1) guide rescue mode, loading system image (CHROOT / mnt / sysimage /)
(2) to re-establish loading sda partition (the install-grub2 / dev / sda)
(. 3) to reconstruct the profile grub menu (grub2-mkconfig -o / boot / grub2 / the grub.cfg)
(. 4) exit bash environment (exit)
(. 5) restart
Experimental operation section
1, first enter cd / boot, enter the startup file, type ls view, you can see there are two grub. (When stored grub boot menu background paper, grub2 is the configuration file) 
2, simulated failure caused by damage grub menu, type cd ../grub2/ enter grub2, ls to see what files. 
3, which is grub2 configuration file, we rm-rf remove it, enter rm-rf grub.cfg, enter ls to see if the file is deleted.
4. After the restart we delete the configuration file, enter grub menu mode that lets you handwritten grub boot menu. 
5, then we restart the virtual machine, press Esc, select the CD ROM, select emergency mode.
6, enter chroot / mnt / sysimae / load system image, this time will find more prefix bash, because you load the system up to the mirror. 
7, the next step is to re-establish the load input partition sda grub2 -install / dev / sda. 
8, the last step to rebuild grub menu, enter grub2-mkconfig -o /boot/grub2/grub.cfg. 
exit after exit bash environment can restart normal operation