Your (so called) mechanic probably re-used your old calliper bolts instead of new ones. He may have over torqued them too. These bolts are under a huge amount of torque and when removed become weaker and so should be replaced. You could sue the garage for this serious safety issue. You should get them to do it properly this time and ask for all your money back. You know it makes sense.. 
You can re-use any of the bolts fitted, neither are stretch type. The caliper bolts need to be coated with thread lock if re-used, the allen key type bolts that hold the carrier can be re-used also.
I think from the picture the allen key bolt has come out, as it appears the whole assembly has pivoted up.
The problem may be it hasn't been tightened to a high enough torque.