First of all if you haven't already take everything out of your trunk
and clean it up well with a solution of water and baking soda.  That
acid will eat up your carpet, paint, and anything else back there.

Secondly, if the battery doesn't work at all you probably need some more
sulfuric acid.  (you can get little containers of same at a good FLAPS
for filling up batteries that are shipped dry)  If that doesn't work,
try to charge it.  If *that* doesn't work, take it back for a
replacement - falling over didn't kill it.

nate

Listen to Nate.  My post would have been even more severe.
You really screwed up.

If you've lost a whole cup of fluid, you're in trouble.  Go to the
place you bought the battery from and get a container of electrolyte
and add it until it comes up to the proper level.  See if it's okay
then.  You may have to charge the battery up first (and you should be
careful about this because you now probably have a mix of charged and
uncharged cells).  

Note, by the way that anything that the spilled electrolyte touched
is going to turn into a corroded horror.  Maybe not today, maybe not
tomorrow, but someday.  Flush the whole area with baking soda and
water, over and over again.  You may not be able to get into every nook
and cranny, but you can at least try.  The sad part is that it may be
several years before the problems really surface so keep a careful eye
on the area.
--scott