LCD shows gray in all modes, cannot take photos

When you put the Mode dail to the SCN mode, It will enter the last used Scene mode. THEN press the MENU button to select other scene mode.
Don't turn any dail. Use the navigation controls left and right from the OK button, to move through the section. Press OK button to save and activate the selected scene mode.
Success

Different scene modes allow for different functions. I rarely use them due to this. You have to set the scene mode to take a normal photo. Meaning in Fireworks scene mode you cannot take macro photos because the lens is focused to infinity already and is locked this way.

You also may have some problems with material in your lens. (I am not sure) If you get your scenes straightened out and it still does not work I would suggest searching for lens error and look for solutions there.

You may have set scene mode for indoor lighting of some type.

You can't. The Portrait mode, like all the other point&shoot modes, are automatic. The camera controls most functions, like exposure metering, auto-focus mode, and white-balance. If you want control over the camera, you'll have to use one of the PSAM modes.

try changing the scene mode. try looking for indoor or candle light modes.
if you can try adjusting the white balance of your camera.

It sounds like the camera shake/blur warning.

You might have the camera set to an inappropriate 'scene' mode. If you are using 'landscape' or 'snow' for an indoor or night shot for example, the camera will select a slow shutter speed which will increase the chance of blur - hence the warning.

If you keep the selector switch all the way to the left (auto mode) you should be fine in most situations. Only use the 'scene' modes for specific purposes.

It means the camera is using a slow shutter speed and may blur.

You might have the camera set to an inappropriate 'scene' mode. If you are using 'landscape' or 'snow' for an indoor or night shot for example, the camera will select a slow shutter speed which will increase the chance of blur - hence the warning.

If you keep the selector switch all the way to the left (auto mode) you should be fine in most situations. Only use the 'scene' modes for specific purposes.

Camera shake (blur) warning.

You might have the camera set to an inappropriate 'scene' mode. If you are using 'landscape' or 'snow' for an indoor or night shot for example, the camera will select a slow shutter speed which will increase the chance of blur - hence the warning.

If you keep the selector switch all the way to the left (auto mode) you should be fine in most situations. Only use the 'scene' modes for specific purposes.

Are you asking about brightness as viewed on the LCD, or are you asking about increasing brightness on the actual image taken? If it's the image, there are two special modes that you can select on your mode dial. They are "Night Scene", "Night Snaphot", and "Indoor". A description of both, along with your camera's other special settings, may be found here:

http://www.canon.co.jp/imaging/mode/camera/scene-e.html

"Night Scene" can be selected directly from the dial. It's the one showing a person standing under a star. Note that Night Scene should be used with a tripod to reduce the potential for blurriness.

"Night Snapshot" works almost as well, but does not require a tripod. It can be accessed by selecting SCN on the dial, and then pressing right or left with the selection buttons until it shows in the menu. The camera will remember this setting the next time you select SCN. I usually keep my camera always set to this setting so that I merely have to select SCN to use it.

"Indoor" works very well in those large rooms that may produce dark backgrounds when taking photos. It also does not require a tripod. It's accessed the same way as "Night Snapshot" (Select SCN, then right/left until selected). This is another good setting that I like to keep sometimes set in SCN.

Try all three settings, (and don't forget all the others too). Think you'll be impressed.

It's the camera shake / blur warning.

You might have the camera set to an inappropriate 'scene' mode. If you are using 'landscape' or 'snow' for an indoor or night shot for example, the camera will select a slow shutter speed which will increase the chance of blur - hence the warning.

If you keep the selector switch all the way to the left (auto mode) you should be fine in most situations. Only use the 'scene' modes for specific purposes.