Backlight not working

The display backlight for the clock is likely burned out, pull the clock and have it repaired.

A faint and barely visible image on the screen indicates that the backlight(s) that illuminates the display isn't powering on. A device called an inverter board takes DC power from the laptop motherboard and converts it to AC power to power the screen backlight.
Before attempting anything below, be sure and remove the laptop battery and unplug the laptop
You need to take the screen apart and check to make sure that no cables are loose that are connected to the inverter board. If everything is secure, the next thing I would do is try to test the inverter board by hooking up an LCD from another laptop to the inverter and turning the laptop on to see if it lights up. If the other screen does light up, then the inverter board is likely good and your backlight needs to be replaced. Backlight replacement, unfortunately, can be very tricky for even an experienced technician, and many will opt to just replace the entire screen.
If you can't test the inverter board, then it might be worth it to order one and see if installing it fixes the problem.
Youtube is a great place to check for instructional videos on laptop repair, such as this one:
http://www.youtube.com/watch?v=jjXLR5yY7X4
Good luck!

Last fishing trip was at night and due to the brilliance of the display my son turnred it down and unknown to me actually turned it off ( the backlight) and it was only after trying everything else found it was the simple solution of turning it (the backlight) back on. oops

I think you are looking at a new display if the backlight can't be replaced.

you should go to settings. display settings backlight and the select how many seconds you want it on for.

A 4 flash indicates you have a backlight problem. Either the backlight inverter boards have failed or you have a backlight that has failed in the LCD panel. You will need a technician to check the set to determine which is the problem.

while the T20 is on and manifesting the dim display....

1. try to slowly close the lid (but not totally closing it) and then pulling it back to its original position slowly. Tell me if the light goes back.

if by doing that action the backlight lights up, you are most likely faced with a Flex cable problem. You might need to buy this from ebay. (dim display problem in T20 series is very common these days due to wear and tear of the flex cable)

2. Also try to use a pen to turn on and off the limit switch that triggers the backlight. This is located above the keyboard below the lcd. See what happens.

There are two different power conservation options that you want to look at. The first is in Settings -> Backlight and can control having the backlight turn off if the device isn't in use. The second is in Settings -> Power which has a setting to turn off the device after x minutes. That could be on, and that is what you are seeing. If the device is actually resetting itself (all running applications close), then you could have a bigger problem and you should contact Dell support.

Reflective LCDs do not have a backlight, because of this they can be difficult to use in some lighting situations. The Transflective LCD can be read while unlit, by ambient light, but it also has a backlight, which makes it better in low light situations.