Canon sd300 cant see pictures taken

Send it to canon,they have a ccd problem,they will fix it free.
just check if this camera is one covered.

This may sound silly, but did you check the shutter on the end of the lens when the camera is on to make sure that the shutter is open? Sometimes some dirt or sand can get into the lens barrel and make the shutter get stuck in the closed position. If this happens, gently open the shutter with a a toothpick or something (NOT your finger because of the oils and NOT something that will scratch the lens).

If the shutter is open and the pictures turn out black both on the camera's LCD and on the computer once you download them, then I'm guessing something happened to your CCD, which is the tiny electronic chip that records the light coming through the lens. Either something inside the camera has fallen in front of the CCD and is blocking the light (unlikely) or the CCD chip has died. If that's the case, I hate to say it but buying a new camera would be more cost effective (and probably cheaper) than getting it fixed.