You should keep a Windows 7 Installation/Repair DVD/USB stored in a safe place to be able to fix Windows 7 boot problems (and not only boot problems). See below how you can create one if you lost yours.
If you lost or don't have Windows 7 installation media you can download an .ISO image from Microsoft using a retail key Windows 7 download.
To put .ISO image to DVD/USB and make it bootable you can use Windows USB/DVD Download Tool.
Having an installation/recovery media you should able to fix most Windows 7 startup problems at least be able to boot to "safe mode" where you could continue with repairing your Windows 7 installation if necessary.
Fix 1: Boot Windows 7 DVD/USB and run "StartUp Repair"
Repeat up to three consecutive runs with rebooting after each run.
If you cannot boot from HDD after this you should apply fix 2 and repeat fix 1.
Fix 2: Mark partition as active using diskpart.exe
Run diskpart on recovery console.
(after boot of DVD/USB on Language Selection screen type "Shift" + "F10" keys to open a command window)
> LIST DISK
> SELECT DISK # ( # - disk number, usually 0 )
> LIST PARTITION
> SELECT PARTITION # ( # - partition number, the partition must be primary! usually 1 or 2 )
> ACTIVE (this marks the selected partition as active)
(this partition will hold boot files in future after running StartUp Repair or bcdboot.exe)
Even small donations appreciated for site maintenance and further development. Thank you.