You have to first determine what exactly siezed. It could have been the piston in the caliper, or more likely, the caliper on its slider pins. If it's just the pins, you can pull them out, clean them (if they're not damaged) and the caliper, and then relube the pins (with silicone grease made for this: high temp) and put everything back together. I would also bleed the calipers to get the old fluid out.
