this is pretty common, the brake has rusted pins that press on the brake pads, when you adjusted it you forced them shut and now the rust has them jammed so they wont release.
take the brake assembly off, remove the 1/4" steel pins, clean the rust off of them and clean any corrosion out of the holes they go in then liberally apply grease in the holes and socket when you reassemble it. the brake will work like new again and then you can go through the procedure to set the brake adjustment