I can't get any pressure to the caliper?

While changing the left front brake pads the foot brake pedal got pushed and one of the pistons got pushed out and the fluid drained out of the dual piston caliper. I cleaned it off and reset the piston back in the housing. Now I can't get any preasure to that caliper. Is there a safety on the brakes that has to be reset or what do I have to do. I reset the computer by taking the negative batt. Termal off for about 10 minutes. What other options do I have?