How to monitor PC in different networks?

My monitor is blank, even though the desktop PC is working, and also, the monitor is working. what's wrong?

• Basically, my monitor is connected to my old desktop PC. When i turn it on, the monitor doesn't show up. But i connected the laptop to the monitor, it work's fine. I also connected the Desktop PC to another monitor, it also worked fine. What could be the problem? I changed the resolution on my working monitor, and replugged it to the non-working monitor, still, nothing happens...What can i do? Just to clarify, the MONITOR WORKS, also, the DESKTOP WORKS.

• Answer:

  What Fox said is true. you need to reset your old PC to 800 x 600 before connecting the monitor. Older PC's don't work too well with flat panel or LCD monitors that are ranged over 800 x 600, sometimes 1024 x 768 is good, too. I got a flat panel monitor that just goes like a TV without an aerial when the resolution is set more than 1024 x 768 when used on a Pentium 3. - Also, if the monitor works on the laptop then set the resolution on the laptop to 800 x 600 for the connected monitor then shut down laptop. Disconnect monitor (turn off mains power prior to, though) then connect to older PC.