There are priors about 0x7b and Windows 10 so I'll only address the No disk available issue.
For this you'll need a working PC to find the Microsoft Media Creation Kit. Use that and answer "this is for another PC" and create the USB stick which you use to do some advanced repair attempts on this unknown PC.
Try the Refresh option fix then the full reset.
Error 0xc000007b system not booting. No disk available, OS was downloaded originally from Microsoft site.