If you don't have a case opener use a ball of rolled up duct tape pressed firmly against the case to unscrew the case back. I bet you a beer the rotor fell off. There is a small clip that holds the rotor on that is prone to popping off if it wasn't seated properly.
If the rotor is off, use a 10x loupe to make sure that when you place the clip back over the shaft and rotate the clip to lock the rotor down you seat the clip fully in the machined groove. Also, if you over rotate the clip the clip will pop off again.
Good luck!