Solve NTLDR is missing using diskpart in command promt

One of the reason behid the error "NTLDR is missing press cntrl+alt+del to restart" is Computer is booting from a non-bootable source or Computer hard disk drive is not properly setup in BIOS.

This may happen if you change active drive in disk management other than the actual drive which has booting file.
How to solve it?

1. Boot into your Windows 7 installation or repair disc.
 2. When you get to the language screen, press Shift+F1

 3. Enter diskpart, then list disk after diskpart is loaded

.

4. Enter select disk (number of the disk the partition is on).

 5. Enter list partition, followed by select partition (partition number).

6. Type active

Now! You can now close the command prompt and reboot your computer.