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)
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