Scenario: you have installed an OS such as Mac OS X on a secondary partition, and after restart you can no longer boot-up your system.
Solution: boot using your Windows cd/dvd; get into the command prompt and type diskpart
Type list disk to see the list of disks installed in your system. Type select disk 0, assuming 0 is the number of your disk containing your partitions.
Now that we have selected the disk, type list partitions to get the list of all partitions on the disk.
To activate the desired partition type select partition n, where n is the partition number.
Now that we have selected the desired partition, we can mark it as active by typing active