My picture is three quarters blacked out on the digital viewer

Time for a new camera perhaps ? Sreen is faulty, most likely and fixing may be too expensive.

Hello, On the C743 this will require that the LCD unit be replaced. This will get very costly real quick unless you can do it yourself. Not much more I can tell you.
Best Regards, Russell

Yes, your screen is dead. You really don't have any options other than to return it to the manufacturer for repair. :(

The problem is with the software in the camera! So take your camera to the nearby service center and do a format of your camera! That will fix your problem. Also check whether there is any problem in ur LCD screen!

You need to go into the setup menu and turn on the display. This is sometimes turned off when you use the optical viewer or in low power modes. Your user manual will show you how to do this step by step.

If your camera got moisture inside and you turned it on before it had a chance to completely dry (usually takes several days) you may have caused a short circuit.

When the big LCD is black the view finder will turn on, right?
You need to replace a new hinge unit. Here is similar solution
How to fix an intermittent display LCD on Canon PowerShot S5IS

please check if the camera is in video mode. Just click on the button to switch from picture view and camera

Thanks for the assistance. It is a problem that the mfg will have to fix.

My viewing screen is black. I had successfully taken a picture, viewed it, and turned the camera off. When I turned it back on about 30 seconds later, the viewing sceen was black. I can 'take' a picure, but it comes out black. Help!