Underexposed image on Auto mode

Do a factory reset, then apply your user customization settings one by one until you find the one which causes this problem.

I can't think of anything that is wrong in your camera. Must be a connection point in a switch that has a too high resistance. And even when you manage to find what switch, you could not replace it.
In the 7 years that your camera exists, camera's have become more sophisticated, and picture quality has increased even in the cheaper models, so perhaps it is worth thinking of a new camera? Perhaps you now even can buy a beginner DSLR, or a system camera, cheaper then your S5IS was 7 years ago.

Success

Please see if the mode forr picture is set to normal, Try to select auto mode and then manual mode to see if there is any change.
If you notice that the grains come on in low light, try pictures in good light to confirm. If still bad the camera needs a check for a faulty imager unit./the CCD might be defective.
Take an estimate to decide.

My problem with my CANON S5IS is that flash doesn't fire under any situation ,even when i press to select any available modes by (MIC/Flash)key, nothing is displayed as it's normal functionality, (AUTO,MANUAL,OFF), the key is working because in some other settings it operates normally; there is a problem with Flash command ,can anyone tell what has happen to flash? thank you all.

If you are shooting using AUTO mode, maybe the settings were changed to a manual modes. If so, check for settings. Flash fires automatically only in AUTO. On manual modes you must force the flash to fire.

CCD FAILURE !

Call Canon for FREE REPAIRS it is a known issue ask them about BAD CCD see this link for service advisory

RATE ME WELL !

Regards

Zohail

Auto and the other settings each have an independent image size that the camera remembers. You need to set the image size for AP. Go to AP mode and press FUNC. The image size should show up in the left menu (it may say L, M1, M2, S, standing for Large, Medium, and Small, resolution settings). Select it and choose a different size that you'd prefer (believe that you want L). Then press FUNC again to exit the menu.

The exposure compensation dial (at the back) doesn't work when you're in Manual. In Manual, you set the shutter speed and aperture to get an image with the amount of over- or under-exposure you need. In the view finder, the "exposure meter" at the bottom shows how much light there is where the lens is pointed. When it shows what you called "2-stops", its really underexposed. Thus your black images. You need to increase ISO, open the aperture and slow down the shutter speed (or a combination of these 3 options)

Set your camera to P or full-auto. Do the photos turn out ok? If they do, then there's nothing wrong with your camera and you just need practise on the Manual mode.

Set camera:

Single shot mode not servo/C modes
Focus sensor set for center
Make sure exp. comp is not set on -(minus) side.
Make sure your subject is locked on and not moving out of the autofocus (center) sensor.

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go to the menu & open setting file.properly check the settings of auto mode