If the generator is running, and it's breaker is set, and the main panel breaker is set, and all your individual breakers are set, then you probably have a loose wire within the ATS automatic transfer switch, which is not that uncommon.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Northwest, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )