The chocolate seized because of either water getting into it or it picked up enough humidity in the air. I work with a lot of white chocolate, and avoid it when possible in the dog days of summer. It'w way more common with white chocolate then milk or dark, and the higher the quality the chocolate, the worse it is.

The water is absorbed by the dry particles in the chocolate and they clump together, and it keeps them from incorporating with the fat in the chocolate, and the fat and dry particles separate.

It can be salvaged with butter, vegetable oil, or solid shortening, and used like you were going to, but it's just as easy to make another batch for the cake and use the salvaged chocolate for something where the consistency sis not as vital. It will still be a bit gritty in the cheesecake (though I've often just crumbled up some oreos and mixed them in too, it hides the grittiness of the chocolate.