Probably the best thing to do is carefully open the box or you might have to cut off the top with a pair of scissors. Pour the powdered dish washing detergent in a plastic bag and break up the clumps with a rolling pin or a heavy spoon. Then store it in a plastic container.
A piece of soft bread works with clumped up brown sugar so it will probably work with dishwashing clumps. Wait a couple of days to see if it worked. The bread will get really hard when used in brown sugar and it makes the sugar stay soft. I did this once with sugar that I thought I would have to throw away because it was so hard I couldn't even break it up with a knife.
I agree with MCW except that after you have got it down to clumps, I would put it in a blender to get it back down to a powder. afterwards, store in an airtight container to prevent it happening again.