Hmm, I've never run into this myself, but it comes to mind that smacking it a bunch with a hammer might work. If that doesn't, I'd just go and get more (perhaps in a smaller quantity).
If you are using it for cleaning, you generally will make it into a paste, so it doesn't really have to be a powder again. I would try to chip away at your baking soda rock. In the future, avoid getting any moisture into your baking soda. That is probably what caused your problem.
I want to use it with vinegar to clean the drains and a paste isn't possible. Think I will just throw it out and get a smaller bag. Thanks
Just add the vinegar to a few lumps you smashed
I had the same problem...I need it for my goats, for cleaning, etc. It got hard as a rock even before I opened it. I just tried this, and it works. Take a pastry blender...the kind with a handle and metal splines....just scrap the top of the "rock", and it turns into powder again.
Put a quarter of an apple inside the box and let it sit there for a day or two, the moisture from the apple should help it soften up a little bit, much like brown sugar.