My screen. Is very dim and impossible to see.

There is a know bug about back light management in Natty. Please look in Bug section.

Thank you for your help.

It seems that you're suffering from bug 765438. If you tell us the brand and model of your computer, that can be confirmed to a reasonable degree of certainty. (You can post that information here, on this question page.) I also recommend you go to bug 765438, subscribe to it (using the green "Subscribe" link on the right side of the page) to ensure you receive notifications when its status changes or comments are posted in it, and indicate that it affects you (using the green "This bug affects..." link near the top of the page).

There are several workarounds for bug 765438, which are presented in various places in the bug (in the description and comments). For your convenience, they are:

(1) https://bugs.launchpad.net/ubuntu/+source/linux/+bug/765438/comments/2

(2) If that doesn't work, you can try the workaround described in the bug's description, i.e., after booting, run this command (in a Terminal window or virtual console) to manually set your backlight to the highest brightness:

sudo setpci -s 00:02.0 F4.B=00

(If that works, you can set up your computer to run that automatically as part of the start-up process.)

(3) If that doesn't work either, you can try using a patched kernel and power manager, as described at https://bugs.launchpad.net/ubuntu/+source/linux/+bug/765438/comments/11.

(4) If that fails, and you're on a 2.6.38-series kernel, you can downgrade to the 2.6.35-series, which does not seem to have this problem (or does not have it as often).

If you have trouble understanding or implementing any of these workarounds, please feel free to post a reply in this question, and I'll provide further assistance as best I can.