|Have you, or is it possible someone else who had access to this computer...|
- dropped it, or dropped anything on it, or otherwise exposed it to a physical jolt ?
- spilled liquid on it, or sprayed it with liquid, or exposed it to liquid such as rain ?
Since the computer last worked properly all the time, has there been a power failure event that happened while the AC adapter was plugged in ?
Those things are the most frequent reasons something inside the laptop has been damaged.
Your problem can be caused by....
- you don't have the network boot option in the correct place in the Boot Order or similar settings in the bios Setup
- you DO have that set correctly but
- the bios is not detecting your hard drive at all
- or - the bios is detecting the hard drive but it's not finding that it's bootable (has an operating system installed on it)
Also - reasons why your hard drive may not be detected as bootable by the bios.
See Response 1: