Answer:
This will *soften* the mass; however it won't turn it back to powder.
1. Microwave it for about 20 seconds. (It's OK even if it's stuck inside a plastic container.)
2. Check it by squeezing the sides of the container to see if it's "giving" a little. Or, stick a fork in it.
3. If it's still too hard, microwave for another 10 seconds and check. Keep repeating until it softens. (Mine did after 25 seconds for a quarter full plastic container of Carnation brand.)
4. Theses small time increments are essential because if you nuke it too long, you'll end up with...TAFFY!
How'd I reach this conclusion? Research, then experimentation. First, I used a couple of different search engines to see if anyone else knew. No luck.
So then I thought, "What's another substance that has a similar problem yet people know how to solve it?" I remembered that this happens with brown sugar, too, & that people say to microwave that to soften it. So I tried same thing here.