Check the manual that came with your monitor, if you still have it. Maybe there are controls that need to be reset. Sometimes you can reset your monitor for more vividness or temperature correction. Also, go into your Control Panel (are you on a PC running Windows) and check your settings there.

If you've already done this, looks like you need a new monitor. They DO wear out, though not usually before your processor or hard drive does.

I've got an older HP scanner that just doesn't scan as well any more due to the light not being as bright any more. Can't change the light in it though.