Did toads evolve from frogs?
"Toads", as commonly used, are not a monophyletic grouping - that is, all toads do not descend from a single common ancestor which only produced other toads. Instead, it's a polyphyletic grouping of those frog species which show common morphological characteristics related to a primarily terrestrial lifestyle. As a polyphyletic group, the parsimonious assumption is that various species of toads evolved from frogs on multiple occasions.

There's several possibilities for similar morphological features in the different toad lineages. The first is convergent evolution - similar selection pressure in the shared terrestrial environment selected for novel generation of similar features. Another is that the common ancestor of frogs and toads had morphological aspects that were a blend between frogs and toads, and depending on environment "toad-like" or "frog-like" features become more prominent or recessive - that is, "toad" genes are sitting dormant in the DNA of most frog species, waiting for the right environment to be expressed. In all likelihood the true explanation probably will be a combination of the two. 

By the way, most toad species still depend on bodies of water, particularly for breeding. For example, the common toad (Bufo bufo) will go to a pond to lay its eggs, where the young toads will spend time as aquatic tadpoles before they metamorphosize into the adult form. So when thinking about toad evolution, it's probably incorrect to view it strictly as frogs compensating for losing access to water (though that might have happened for some species), but rather toads evolving to take advantage of the opportunities presented by a terrestrial habitat that's coexisting alongside watery environments.