Reinstall GRUB2 to the MBR, either:
* This should fix the problem even though in this case it was not caused by a recent installation of Windows, see below.
You're installing alongside Windows 7 (not Windows 8 or later) and there's no reason to believe this is a UEFI system (no "Secure Boot"). Therefore, we should expect that the cause of this problem is the usual cause of Ubuntu failing to boot after being installed as a fully-fledged (i.e., non-Wubi) system.
Sometimes Ubuntu's installer will not successfully install the GRUB2 boot loader to the Master Boot Record. As I answered in this related (but somewhat different) question:
While most installations go without anything going wrong, this is
actually one of the more common installation problems. It can occur if
you manually tell the installer not to install GRUB to the master boot
record of the first hard drive, if it identifies the first hard drive
incorrectly (its idea of first isn't necessarily what the BIOS tries
to boot first), if it thinks a suitable boot loader is already in
place (and is wrong), and possibly for other reasons.
Reinstalling the GRUB2 boot loader to the master boot record of the
drive that the BIOS is trying to boot almost always solves this
problem. When someone with a working Ubuntu system installs a Windows
system alongside Ubuntu, the Windows installer will overwrite the
master boot record with its own boot loader, which does not detect and
provide an option to boot Ubuntu. That's what your situation has in
common with that of users who've just installed Windows beside Ubuntu.