This is not the issue with the monitor, it's an issue with the computer.
The monitor is going into power save mode because it's not getting a video signal.
Check the video cables on the system.
Does the PC make noise when you turn it on? Fans spin, hard drive spins, can you hear the Windows startup sounds? Could be an issue with the motherboard, RAM, video card, power supply.
Check the motherboard for any bad capacitors, they should be flat, if they are puffy or have stuff leaking from the tops, need a new motherboard.