This is helpful Excerpt:
At the grub prompt try these commands:
Now, suppose the output of ls is: (hd0) (hd0,msdos2) (hd0,msdos1), in order to find the linux root filesystem run:
ls (hd0,1)/ which should give you all the files/folders in / such as bin/ boot/ cdrom/ dev/ etc/ home/ lib/ etc.
Once this is done continue with:
linux /boot/vmlinuz-3.13.0-29-generic root=/dev/sda1
Of course replace vmlinuz-3.13.0-29-generic and initrd.img-3.13.0-29-generic with what you find in your /boot
/dev/sda1 is the location of the root system. You may have to change it if /dev/sda1 did not work.
Run those commands
grub> linux (hd0,1)/vmlinuz root=/dev/sda1
grub> initrd (hd0,1)/initrd.img
Replace hd0 with your hdd number and /dev/sda1 with your partition holding the /boot mainly the dafult will work if you don't have more than one hdd and with default installtion
When you can boot again you shoulf fix your problem
Here is another one => repair-grub2-when-ubuntu-wont-boot/
And here => repair-and-reinstall-grub-boot-loader-in-ubuntu/