Spackle is used frequently to patch holes and give drywall an even surface. After spackle paste is applied, it will harden and become part of the wall. It is no surprise that the paste will eventually harden in its container, as well. If this happens, you can add water to it to loosen it up so it can be used again.
Add a small amount of water to the spackle paste. Determine how much water is needed based on how dry the spackle seems, but always go with less because you can add more water later.
Mix the water with the spackle with a power paint mixer. It has a long stem with the agitating apparatus at the bottom.
Add more water as needed if the paste is still too dry or hard. The paste needs to be thick, but should move smoothly in the bucket, like peanut butter.