|Your system is configured to boot off a network...no network is present therefore you're getting the error message.|
It could be that your CMOS battery is bad & the BIOS reset, thus changing the boot order. OR it could be that your hard drive died & your system is moving on to the next device in the boot order which apparently is the network.
Assuming you have a floppy, CD & hard drive, change the boot order to floppy, CD, hard drive. If there's another setting similiar to "other devices", disable it.