It might be that the main OS partition's boot flag was set off. If that's the case, try the following to see if it fixes the problem:
Preferably from a clean computer, I need you to download: gparted-live-0.10.0-3.iso
Create a bootable CD for Gparted from the ISO image. You can use ImgBurn
Now boot off of the newly created Gparted CD.
You should be here...
By default, "do not touch keymap" is highlighted. Leave this setting alone and just press ENTER.
Choose your language and press ENTER. English is default 
Once again, at this prompt, press ENTER
You will now be taken to the main GUI screen below
Is "boot" next to your OS drive?
If "boot" is not next to your OS drive under "Flags
", right-mouse click the OS drive while in Gparted and select Manage Flags
In the menu that pops up, place a checkmark in boot
like the picture below:
You should receive a small pop up like this:
Choose reboot and then press OK.
If the above doesn't work, there's one more method I have in mind.