Just had my brake pad fall off

I’m a rca with only a couple of weeks experience but how as a regular can you let it get to this, Like come on it’s a metris with only 65000 miles how do you let it get to me leaving the post office with a hard grinding sound and as soon as I try to break the entire break pad fell off the rotor leaving me without a vehicle and no breaks while trying to make it back to the parking space.