This message means that your system is trying to start up by connecting to a network instead of the Hard Drive because it can't see the operating system on your Hard Drive for some reason. There are several reasons why this message could appear:
You want the system to use PXE but the the ethernet cable is not connected, or network is not available.
The boot sequence in the BIOS has been changed and is currently looking to PXE instead of the Hard Drive.
If you have recently made changes inside your system and have accidentally knocked, loosened, or not reconnected the Hard Drive cable on the Motherboard or the Hard Drive(s).
The Hard Drive has failed (faulty) for some reason.