Please take the following educated guesses with a grain of salt. I may be wrong.
A backup won't move / disable your boot drive. It seems more likely to me that you manages to image the drive to the external drive, and then the boot drive died. BTW, what software did you use to make the backup, and is it an image backup or a clone? Was the external drive bootable before?
You can test the failed internal drive assumption by one or more of the following methods
■Remove the external drive. Boot to BIOS and see if the internal drive is detected. If it is, try to boot to it.
■Remove the external drive. Boot to a drive utility if you own a bootable drive utility, and examine the internal drive. If it's dead, then it's dead.
■Boot to the external drive, bring up the Disk Manager, and examine the internal drive to see if it is detected or dead.
■If you can see the internal drive, you can try a repair install of Windows.
Congratulations on getting a backup done in time. If the internal drive has failed, instead of some transient file corruption, you will have to buy a new one, replace the dead one, and then copy your saved image back to the internal drive. My guess is that this is the most likely scenario.
Let us know how it goes.