As pressure was released the brake pads moved. Wrap a cloth round the master cylinder, take the lid off the reservoir then push the pads back. It's better to do it by hand as you don't want to damage the brake material. Failing that, take the pads out (this is a good chance to grease the pins) and push the cylinders back. If they are really bad you can buy screw driven jacks to do it, two plates that go in the gap and are pulled apart by the screw. Next time you change brake pads, save the old ones so you have a pair you can use a screwdriver to push back and make yourself a wood or steel spacer to hold the pads apart while you have the wheel off.
Once it's all back together pump the lever a few times, it'll take a few full length pulls to get the pads back touching the disk.