Network boot

Recently I am facing a problem with my HP Probook 450 G4. Some days ago when I powered it . It started with beeping and showing this ( attached image ). I checked internet to solve and found . They said to Disable Network Boot from BIOS. I did it as they said. But after 2 or 3 days I am facing beeping problem again. When it starts IT SAYS ENTERING NETWORK BOOT and just after starts BEEPING. I check BIOS but there Network Boot option was disabled as i saved it before.Please tell me a way to solve the problem.