I've had salt get pretty solidified on me, but never to the point of having to smash it on the floor. Usually some scraping/bashing with the measuring cup, gets it into small enough pieces that I can dissolve by hand into the water.
If the salt is that far gone, and this is for a reef, I could consider/recommend just tossing it. What has happened is that moisture has gotten to the salt, and the calcium carbonate has precipitated. Basically why you add salt to water, never water to salt.
Either way, if you decide to use the salt, or for the future anytime anyone uses clumped salt, always test the alkalinity after it's mixed.