Computer won't boot "Reboot and Select proper Boot device or Insert Boot Media in selected Boot device and press a key."

everytime i turn on the computer a black screen pop up saying
"Intel UNDI, PXE-2.1 (build 083)
Copyright (c) 1997-2000 Intel Corporation


This product is covered by one or more of the following patents:
US5,307,459, US5,434,872, US5,732,094, US6,570,884, US^,115,776 and
US6,327,625


Realtek PCIe FE Family Controller Series v1.27 (10/31/11)
PXE-E61: Media Test Failure, Check Cable


PXE-M0F: Exiting PXE Rom.
Reboot and Select proper Boot device
or Insert Boot Media in selected Boot device and press a key."
i can't even do anything on the computer and when trying to use the media disk it stays on a black screen.