Why does my circular saw motor turn but the blade does not?
It is very likely, that the fixating screw/disk is loose. Do not operate the saw in this state. It is dangerous.

A circular saw usually has a direct drive. I.e. the motor spindle is also the center shaft for the blade. So if your motor is spinning, but not the blade, it means the blade has lost contact to the shaft. 

Check if the blade is properly centered and aligned with all locating pins (if your saw has some). Then fixate the screw using the appropriate wrenches.