You can go to Cummins/Onan and download the trouble shooting guide if you don't have the manual.
First step, check for power on the line side of the CB.
Second step check for power on the load side of the CB.
Third step check for power on the line side of Automatic Transfer Switch.
Forth step, check for power on the load side of the ATS.
Fifth step, CB panel should be the next load to test.
If you can follow these instructions, eventually you'll find the open. If you have problems following these steps, hire someone who can follow them.