If the clay isn't completely dried out you can reconstitute it pretty well
by taking a pencil and poking lots of holes through the bag and into the
clay. After poking the holes put the entire bag of clay into a bucket of
water and let it sit there for a couple of hours. This re-moistens the clay
and you can rework it.
If the clay is entirely dry, you can turn the clay back into slip by
breaking the block up into chunks and putting it into a bucket of water.
The slip is then dried out on an absorbent surface and re-wedged for future