|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:
The bios will attempt to boot the computer from a USB connected drive if the Boot Order or similar settings are set a certain way in some bioses.
If the USB connected drive is not bootable, the bios MAY NOT try to boot the computer from anything else.
Unplug any flash drive or external hard drive to prevent that from a happening.
Flash drives are nevver bootable unless they have been prepared with software to make them bootable.