There are several possibilities for this problem.
This can be broken down into two catagories; Hardware and Software.
The possiblilities under Hardware are that your monitor or your Video Card could indeed be going out.
The possibilities under Software are that your Video Driver or one of it's associated files may have been
3. conflicting with another driver
4. become corrupted
the settings, such as Gamma, Sync Rate or Brightness may have been accidently changed either manually or by another application.
Unfortnantly, in these days of automatic updating driver problems are becoming harder to diagnose.
Let's try changing the settings in your software first though.
If your running one of the Windows 32 bit operating systems (such as Windows 95 or above) you can RIGHT click on your desktop to bring up a popup menu, then select Properties.
Then select Settings tab, then click on Advanced and find the Gamma Curve or Gamma Ramp (same thing, different versions of Windows, different Names. Go figure).
Adjust your brightness and contrast accordingly.
If this dosen't work you have a more serious problem.
For this you may want to contact a local shop.
Be prepared to answer the following questions:
1. How old is the machine?
2. What make/model is your machine?
3. What Operating System are you using?
4. When did you first notice the problem?
5. Is this the first time you have encountered the problem? or does it come and go?
6. Did the problem appear suddenly or has it progressed slowly to this point?
7. Did the problem occur during use? or at bootup? or do you not turn your machine off?
8. Did you encounter the problem after servicing the machine? (either have work professionally done, or install something yourself) (this could indicate a loose video card, dirty card connection or possible short across traces on the motherboard or video card)
9. Did you encounter the problem after installing a program or update? (this could indicate a driver problem, such as a conflict or corruption)
10. Does turning the computer and/or monitor off for long periods of time (such as overnight) seem to cure the problem briefly before the problem re-appears? (this could indicate a thermal problem).
One more thing to consider is buildup of dust inside the machine. Most household dust is electricly conductive and can bridge (short) solder points on the motherboard and cards.
It is reccomended that you open your box at least twice a year and vaccuum the boards as well as remove the faceplate and vaccuum the air intakes to prevent overheating.
If your not comfortable doing this yourself most shops will be glad to do it for you.
I just realized I forgot to answer your second question.
Whether to turn the monitor off or not depends on how long your going to be away from your keyboard (or AFK).
In a setting such as an office environment where you are only going to be away from your desk for less than a couple of hours it is better to leave the monitor turned on. It's rather like going to sleep then waking up again. The roughest time any electronic device has to endure is the sudden rush of energy followed by the thermal stress of cold components having to heat up (or cool down when turned off).
If your going to be gone from your computer for more than a couple of hours, you can let your monitor take a nap.
~ Raven ~