Help! How to unlock or unfreeze brake calipers?

My husband was changing the front brake pads on my 96 Toyota Previa van (which he has done many times before) and the caliper on the right side froze up and wont move. We bought the pads from the auto supply store and the book says they were the right pads but is seems the calipers wont fit around them. Anyone know how to unlock calipers? He has tried spraying silicon spray on it and did not work.