This question already has an answer here:
I have recently dual booted ubuntu 14.04.2 LTS alongside window 7 in my dell laptop from live usb flash drive.
The dual boot was working until I deleted empty partition (not ubuntu or window partition) and merged it with anouther. When I restarted it displayed the following:
error : unknown filesystem. Entering rescue mode... grub rescue>Did I mess with my OS or what? How can I recover to have dual boot again?