A grain of sand is actually a very small rock.
It is very possible that it has jammed the lens carrier.

Error 18 is Canon's code for a lens jam.

There has been a lot of threads about Error 18.

Several people have been successful in clearing a lens jam. I suggest you read all of one of the longer of the Error 18 threads and see if their suggestions will help your camera:

http://reviews.cnet.com/5208-7813-0.html?forumID=58&threadID=56917&messageID=675931

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