Hi,
Identify the PWM IC. Check if you get a pulse at the PWM IC output. Check the pulse with an oscilloscope and see if it seems fine. Ensure the frequency seems fine.
Check the MOSFET/IGBT devices and check if there is any short across them. Check resistors and/or zeners near the MOSFET/IGBT to see if any have burnt out.
Check the output rectifier(s). See if you can detect a short-circuit. Check the output capacitors to see if they have leaked or not and check the capacitance and compare against the marking on the cover of the capacitor(s). Check inductors to see if any have burnt out.
Check the PCB traces to see if anywhere the traces are burnt.
Check the feedback circuit, ie, the shunt-regulator and associated components.
And lastly check the fuse to see if that fused out. Check the input capacitors.
Hope this helps.
Tahmid.