Try one of these

listed from least drastic to worst case:
1. Push it through a sieve
2. Roll it out w/a rolling pin
3. Steam it in the top of a double boiler
4. Put it, in its bag (but not box) in a 350 oven. By the time
the bag is warm, the sugar s/b softened or delumped.
5. Put it thru a food processor (be careful on that one-it's
possible to turn it into superfine sugar if you aren't watchful),
blender or meat grinder
6. Put a wedge of apple into the box or bag of sugar & reclose it.
Microwave on high 20 seconds per cup of sugar. Let it stand for 5
minutes. Repeat if necessary. (I've used this on brown sugar)
7. Give up & melt it down over slow heat on the stove. It makes good
syrup. Add extracts like vanilla, maple or butterscotch. You could
also use the sugar syrup to make fresh lemonade.

Hope this helps.