Did you reinstall windows 7 on the same partition?
If so, then the problem is likely that stated above. Any windows installation overwrites the master boot entry without asking or informing the user so you will just need to reinstall Grub.
Open a terminal in Ubuntu and run the command: sudo fdisk -l (lower case Letter L) and check the output to be sure sda2 is an ntfs partition. If you don't understand the output, post it here and someone should be able to help.