The problem could be dirt, debris or a dislodged lens. Here is a great site that may help you getting it cleaned and working again. Of course there is also the possibility that you truly have a lens error.

"ZOOM LENS

ERROR

Movement of the lens barrel does not end within the specified time.

Cause and Probable Faulty Part:

MAIN PCB ASSm'Y

OPTICAL UNIT"

Now if you are inclined to fixing your camera, you can download the service manual from here or use the guides from here Canon PowerShot SD750 Good Luck.