My Sony Monitor SDM-HS73 comes up blank when I boot my PC. When I connected the monitor to another computer it gave me a message "out of scan range 46.7khz-87hz" It was working fine and one morning it started acting like this. Do you have any idea what is going on and can you help me please?

When you start it up do you see the first screen where it shows Hardware specs and other stuff?

If you do...
Try booting into safe mode. What probably happened is your resolution was changed to something either your monitor doesn't support or your video card doesn't support. To boot into safe mode press F8 once you pass the first load screen. F8 will take you to an options menu and a list that's like this.

-Safemode
-Safemode with Networking
-Safemode with Command Prompt

There will be more options than that but you only need to worry about the first one. Select "Safemode".
This will boot your computer with the bare minimals of what need to be run. Sometimes it takes a bit longer to start up than a normal boot so don't worry if it takes awhile. Once the computer is booted up you'll need to change the resolution of the screen. Do this by..

Right Click on the Desktop
Select Properties
Click on the Settings tab
Under the Settings tab you will see a section that says screen resolution. Feel free to change it to something lower on the scale. 1024/768 will probably work fine. Once you select this your screen will flicker and it will come back with a Yes or No quesiton. This is just asking if you want to keep these settings. Click yes and reboot your computer.
If this fixes it congrats.