|Some monitors will display a message "signal frequency is out of range", or some other similar message, whenever the monitor is on, and the monitor is NOT receiving any video signal, after a short delay.|
(For some cheap monitor models, e.g. some Acer ones, you may briefly get that message while booting even when nothing is wrong.)
If that applies in your case, you will probably get the same message when the monitor is switched on when the video cable is not connected to anything.
You haven't mentioned anything, so far, that indicates your computer is actually starting up, and if it isn't, you would probably get that "signal frequency is out of range" message, when the monitor is connected to a video port that should produce video.
Is the power led on the computer lighting up?
Can you hear that the CPU fan and power supply fan are spinning?
Is the hard drive activity led blinking like it usually does while booting the computer ? (If you normally need to logon to Windows, it will stop blinking shortly after the boot gets to that point.)
If those things are all okay, does your desktop computer have both onboard video (built into the mboard) and a video card in a mboard slot ? If so, you will probably get no video from the video port for the onboard video - the one in the same I/O area as most of the other ports on the back of the case. You must plug the monitor into the video port on the card in a mboard slot.
No video is a common symptom of a malfunctioning system. Usually there's nothing wrong with your monitor or the video adapter - something else is wrong.
E.g. Failing or dead power supplies are common.
When they are failing, leds may come on, fans and hard drives may spin, yet the computer will not boot all the way. The hard drive activity led either does not come on at all, or it comes on and stays on.
Have you just been messing with the ram ?
Installing ram that is NOT compatible with using it in your mboard, or a poor connection of the ram in it's slots, can also cause it to NOT fully boot.