1998 Ford Mustang Battery still on when key is out

I just recently replaced my break pads and a rotor and everything was running fine when randomly one day I put the key in the ignition and the battery turned on the dash lights but the key would spin freely without any resistance. When I took the key out the battery was still on and it died after I had to walk home. I replaced the ignition switch but that didn't do anything. I think there might be something wrong with the starter/solenoid.