One of the common problems in Linux installations is to lose or damage the the boot loader (Grub), specially if on the same machine coexists other OS from Microsoft family.
On the other side, while you are testing Linux you can loose your Windows Boot.
I used to repair this kind of problems in the old-school way: Booting a Linux liveCD+mount HD partition+chroot+install grub+cross fingers. But this is not a method recommended for novice users…
Recently I’ve discovered an utility that is designed specially for this! The utility is called Boot Repair CD (you have to burn the ISO to a CD or USB) and after booting, an assistant will appear and magically solve your boot problems
Even they will help you if the utility can’t repair your boot area:
Launch Boot-Repair, then click the “Recommended repair” button. When repair is finished, note the URL (paste.ubuntu.com/XXXXX) that appears on a paper, then reboot and check if you recovered access to your OSs. If the repair did not succeed, indicate the URL to firstname.lastname@example.org in order to get help.
For advanced users, it also have a bunch of options related to Grub and MBR:
Give it a try!