First you will need a DVD with the windows OS. After you boot from the DVD access the “Repair your computer” link. After the open the Command Prompt:
After you open the Command Prompt enter the following commands:
diskpart list disk select disk 0 list partition select partition 1 active
NOTE: The selected partition must be the partition on which the OS is installed. In order to mark a partition as “active” the disk must be MBR, so in case you have GPT you must first convert it to MBR(use Aomei Partition Assistant).
After you execute the commands restart the PC and boot again from the DVD. This time execute the following commands from the Command Prompt:
bootrec /fixmbr bootrec /fixboot bootrec /rebuildbcd
Now your MBR should be fixed and you should be able to boot into your Windows OS.