Resizing root partition in LVM broke Grub

I'm using Debian Wheezy hosted on W7 in Virtualbox. I needed to enlarge root partition then added a new virtual disk to virtual machine. Doing the steps mentioned below brings me to grub prompt. I:

• Added new virtual disk

• Formatted and mounted

• Created PV

• Added to same VG with root partition

• Extended root partition LV with lvextend

• Resized the file system.

Everything went well and I checked that with df command and lvscan but after reboot I couldn't boot the system. What point have I missed and how to recover the system?