A first step would be to boot into the Recovery Console:
http://www.windowsnetworking.com/articles_tutorials/wxprcons.htmlWhen on the RC prompt, type:
fixmbr
and hit enter.
Reboot and check.
If no good, you may have to run TestDisk:
http://www.cgsecurity.org/wiki/TestDiskIt must be started from a bootable CD. I recomment Parted Magic (
www.partedmagic.com), which already has it included in its System Tools menu.
Here's a step-by-step guide for using TestDisk:
http://www.cgsecurity.org/wiki/TestDisk_Step_By_Step