It came out, that BIOS has loaded the default settings by itself. Therefore SATA was disabled and it couldn't find my HDD. Therefore it was trying to do a network boot etc.
We have a small network, we are using DHCP that gives us IPs like 192.168... But today when I started one of the computers there is a message:
For Realtec RTL8139 (A/B/C) RTL8130 PCI Fast Ethernet Controller
Client MAC ADDR: 00 11 09 97 55 33 GUID 00000000-0000-0000-0000-000000000000
DHCP... (and fast changing symbols \|/-\|/-... that looks like spining)
After a while the message
PXE-E53 No boot filename received
PXW-M0F Exiting PXW ROM
apears at the bottom and the same goes again. If I unplug the network cable it shows a message, informing me that the cable is out. What should I do?