hey, the pad might have just fallen out also, ive seen that happen before, it either just breaks and rusts through or it just wears down enough that it slides out(but that would be a lot of wear haha) but check the caliper to make sure it did not rub into the rotor, or that you did not push the piston out of the caliper.

 

so far, i have put on both new calipers(and brackets) all were froze. i shouldve checked this before i bought the car. but i changed those, pads, out new fluid and a gasket in. and then i found ou that the rear end is out an 81... was a fun day to autozone. lol. and now the piece wont start... i do believe it is either the solenoid switch or connections. im avout to go out to the garage now

 

oh yeah, i changed both rotors too

 

i have around 300 into it so far. but at least it looks good right? lol