Sometimes when you open your computer it will continuously restart automatically and doesn't run. If such type of error came then follow these below instruction.
1) switch on your computer and press F8 key
2) choose " disable automatic restart when system failure "
3) After that computer will show blue screen error where a) unmounted volume disk
4) choose bootable disk which OS you are using and after boot from your disk there is option to " repair your computer" choose it and solved
If my suggestion didn't work then please feel free to comment
Note: You can use windows XP bootable CD for solve unmountable_boot_volume error
Note: You can use windows XP bootable CD for solve unmountable_boot_volume error