PXE-E61 – Media test failure check cable

I have a brand new P4 1.6Ghz, 128Mb, 40Gb Seagate Hdd, Win 98

I did a normal shutdown, booting up the next morning I got the following :-

Invalid system disk
Replace the disk, and then press any key
I checked for any disks in the CDR & Stiffy nothing pressed any key and got the following

Intel (R) Boot Agent Version 4.0.19
Copyright (C) 1997-2001, Intel Corporation

Intel Base-Code, PXE-2.1 (build 083)
PXE-E61 : Media test failure, check cable
PXE-Mof : Exiting Intel PXE ROM
Boot failure : System halted

After reloadind Win 98 from my rescue disk the problem is fixed

This is the second time this has happened but with different PC’s

What and Why did this happen ?