There are other reasons for that error message.
I was getting that every once in a while on one PC and with coincidental irony I got it again yesterday.
Normally for unknown reasons your Bios on the MB changes the boot order device meaning your C drive has been demoted and another hard drive, DVD rom or even floppy has been put in the number one boot position and because there is no Windows OS it says it is missing the NTLDR.
Click restart then when you see your first MB page starting tapping Del key.
It will show the BIOS screen
Use the right arrow to move over to boot and once in there look to see if your C drive is in the first position.
If anything else is in that first position click on that object then scroll down and click disable and keep doing that to other crap until your hard drive is in the number one position then click F10 and save the changes.
However, you may have a failure and it could be the MB or more likely the Hard Drive and whatever if you get it started back up everything to another drive.
It could also be something as simple as a glitch caused a fragment to be erased in the wrong spot by bad luck and you would have to repair it or reinstall the entire OS a guessing game and frustrating
If that does not work order a new hard drive or buy one locally, disconnect your current HD and install your OS on the new drive.
After that is done reconnect your old drive and turn it on. your old drive will be a slave and some programs will have to be reisntalled some may work, try the start icon in each program folder to see before reinstalling every one.
My one PC did this on and off and I would go in and reset the boot priority but yesterday it did it and would not stop. So, I took an old hard drive and formatted it and installed XP on that then reconnected all the other drives and everything is there I just have to reinstall a bunch of stuff, no fun but the data is not gone just out of reach for the moment.
this can mean your PC is on its last leg and if you get or build a new PC just make the current HDs slaves (They now becomes slaves automatically)
LMK if I was not clear about anything and I will try again
Wulfmann