The grub rescue> prompt comes when grub is missing. Some times if you have dual boot system and you delete any partition contains win 7. To rewrite the partition table when you reboot, you can stuck with this grub rescue prompt. Here you find no commands works, So dont panic follow these steps, and you will get your lost child
Step 1. List the partition you have on the disk
Step 2. The above command display some partition and from these partition find out which has /boot/grub,Which contains several .mod files.
grub-rescue> ls (hd0,5)/boot/grub
If you will find some files, it means this disk contains the grub file, else search in other partitions.
Step 3. set this partition
grub-rescue> set prefix=(hd0,5)/boot/grub
Step 4. Now insert module on kernel using insmod (install loadable kernel module) command.
grub-rescue> insmod normal
Step 5. Restart the machine
Now after the machine boot in linux update and install your grub
Open a terminal and type below commands
$ sudo update-grub
$ sudo grub-install /dev/sda