My camera is not showing through Viewer or display screen, not able to take pictures...... My display screen works fine to view old pictures taken..What could be the problem?

Check to make sure that you did not accidentally change image format from jpg to crw(camera raw). Your camera lcd will only display jpg files and you need special software to view raw files on the pc.

If your Canon A80 camera shows previously stored images of memory card in playback mode but does not show live view in camera mode to take fresh picture then ccd imager chip got defective and need to be replaced.
Local camera repair shop will help you to replace ccd imager chip because camera will take apart in detail and soldering work is needed.
Thanks.

Think that you might have accidentally pressed the DISP or DISPLAY button on the back of your camera. This manually turns the LCD display on and off to save batteries. You would then use the optical viewfinder in its place. To turn the screen back on, just press the DISP button one more time.

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.

Not to worry, this a very simple problem. Your display is set to off. All you need do is set it back on. And this is how you will do it.
To the right of your screen where you have your buttons, look for the display button. Below it is usually labeled DISP. Press this button again and again and watch as the screen display changes. At one of those instances it goes blank/off, but at the others it displays the view in deferent manners. Stop at any of those display options where it shows the view.

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

So camera doesn't give live view to take picture? and you can view old / stored  pictures OK, is it?
If yes, then it is problem of CCD imager in Canon A80 and this part needs to be replaced.
Canon will repair your A80 free of cost as per announced scheme of CCD imager support.
Click Canon CCD Advisory to know how to get free repair facility.

This setting is just showing you what part of your image that will be blown. Just use your arrow button to scroll thru other settings till your screen goes back to normal.

Think it might be your batteries. 700mah batteries are pretty weak by today's standards (most are at least 2,500 mah these days). I'm guessing your batteries are several years old? If so, they're really only meant to have a usable shelf life of a few years, whether they are used or not. Recommend trying a new set of rechargeable batteries.

This sounds like CCD sensor is faulty and should be replaced . Camera works fine in playback mode generally with this problem but not in capture mode ( camera has gone 'blind'). Should be repaired free of charge if the camera is as old as you say.