You can take the lever+shifting mechanism off the body, this will make it a bit easier to clean and make sure there isn't any sand left when you are done. If you tried shifting too much after you fel chances are your shifter is damaged, the sad thing is that you have to replace the whole shifting unit
(if you can find a deal on a whole new shifter this might even be cheaper, I think the replacement part costs about 120 euro, but don't quote me on that)
Try cleaning it as good as you can, an airgun wil help a lot (high pressure air) . Remember to regrease everything when you are done!
Hope you get it to work properly again, exactly what happened to you is why I don't like shimano handles for a cross bike. The way they work makes them real dirt eaters, and no small parts can be replaced to make it work again relatively cheap. (as you can guess I switched to campagnolo, but I still think the shimano levers shift a bit better when your bike is really dirty. You can apply more force when you shift up because of the bigger lever. I also like the shifting cable sticking out the side, it helps you not to slide over the levers when they are wet and slippery. This shouldn't be a problem when you race, but if you only ride for fun you can't switch to a clean bike every 3 km