When you say that you "log on into your computer," do you mean that you're logging into your OS and then the screen goes black? Or that you're powering on the machine and not getting any video?

You may like to check the brightness settings and see if you can turn those up manually on your monitor. if you are getting all the way into the OS before the monitor goes dead then there may be a brightness setting somewhere in windows that needs to be fixed.

When your monitor comes on does it come up with any messages such as No Input or something similar?
Also have you added any new hardware to your computer recently and if so what was it?

If it actually gets to the login screen before it crashes it could easily be the video drivers. When you restart your computer press F8 and boot into Safe Mode and see if it works fine in there.

Also have you tested another monitor on the computer.

If the monitor itself is the problem than there's not much that can be done.

Yes I agree with Nuitari's it could be the pixels out of range. Try reinstalling your video driver.

When the computer starts press F8
it will give you an option for Safe mode.

Press your Windows key on the keyboard and hold it down then press pause break. This will access your Device manager. Remove all Video devices. You might even want to remove you monitor driver as well. Then restart. It should automaticly reinstall the monitor driver and the video card. If not you can find it on your restore cd or on the manuf. website.

Hope that helps. But yah if you can see the post initially and windows starting up its a driver or resolution issue.