The output of Ic93 is a pwm signal, off state is about zero volt, on state is about VCC.
Q901 is used as switch, off state zero Volt, on state voltage repents on the properties of the mosfet, let say at least 10V.
You could check the resistors R914 and R914, maybe they are open.
IC93 pin 2 (Zero Crossing), pin 3 (feedback), pin 4 (Current Sensor), should be at zero Volt, maybe IC93 shuts down when there is no zero crossing signal detected, Check R915, D904.