Can't take any photo with my Canon S3IS any more. The view finder and the screen is black when I try to take some photo but I am able to see all photo that is already on my memory card in the view finder & the screen. WHY?

Look into the lense while setting the shutter speed at a half second or so (slow enough for you to see the shutter open and close) and click the shutter release... you should see the shutter open and close. If you don't see this, then you may have a 'sticky shutter' resulting from lubricant seeping into the shutter/aperature mechanism and preventing the shutter from functioning as it should.(Common problem reported with the S2 and S3 cameras... and probably the S1 as well)

I have gotten my S3IS to take pictures by turning on the power, then opening the battery compartment door and reclosing it repeatedly until the shutter opens (from the power surge) and an image appears on the LCD.

After confirming the sticky shutter was indeed the problem, I pursued a more permanent fix.