My canon rebel

SOURCE: Canon Rebel will not shoot in AF mode

the mirror is problem of your camera,it alway been close that why the sensor is functioning properly cause that sensor is working in af mode....check the post at the side of the mirror if not broken and the spring of the mirror if not dislocated....doing this you should removed the lens from the body......

SOURCE: canon eos digital canon rebel xti - LCD won't work when shooting

Here is the Link to the manual
If your eye nears the vewfinder
the lcd is automatically shut off
this setting can be disabled p101

Page 95 below shows how to
wake up the camera and adjuat
the brightness


http://www.retrevo.com/s/Canon+Digital+Rebel+Xti?rt=pd

SOURCE: canon eos digital canon rebel xti - LCD won't work when shooting

Most DSLR cameras (like your Canon) don't have a live view - in other words, you can't "see" the image before you capture it. You must use the viewfinder.

SOURCE: CANON REBEL XTI LCD

Your battery is low ...shut the camera in off position and replace with fresh battery

SOURCE: Cannot preview shots in lcd with new Canon Rebel xs 1000d

There are two modes to the XS's Live View system, selectable via Custom Function 7. Live View is disabled by default, by the way, so you'll need to find this menu item in Settings Menu 2 to turn it on. The two options are called Quick and Live modes. Quick mode works as most Live View modes do, dropping the mirror when you press the AE-Lock button (marked with an asterisk) to use the conventional phase-detect autofocus system. This temporarily blacks out the Live View image, which makes it impossible to know that you're still framing the subject and holding your AF point in the proper place.
That's why they added the second mode, which they call Live, but is better described as contrast-detect autofocus. This works just like a digicam, reading the image from the imaging sensor while adjusting the lens for the setting that produces the most contrast in the image. A small square appears in the center of the image area, and you can move it around with the cross keys. Just like Quick mode, unfortunately, you have to press the AE-Lock button first until you hear the focus confirmation beep, then press the shutter button to fire. It's a little slower at times than phase-detect mode, but it gets the job done. Live mode is really better for tripod work, where you can place the AF area right where you want it and confirm focus onscreen.
Both modes offer the ability to zoom in on your live view, excellent for confirming focus before capture. Using the Magnify button just right of the AE-Lock button, you can zoom in 5x or 10x, then press the AE-Lock button to focus if you like; but of course you can't confirm framing in that mode.