the default state of an automatic transfer switch is GENERATOR. when you plug into shore power you should hear a loud THUNK sound caused by the ATS electromagnet switching FROM genny TO shore. it may take 5-10 secs for the EM to switch. conversely, when unplugging from shore power you should hear the same THUNK of the EM back to genny.
since shore power is working unplug and listen for the THUNK sound. if you don't hear it you either don't have an ATS or it's stuck on the shore power side. sometimes you can unstick the ATS by lightly tapping on the ATS with a rubber mallet. if you do hear the THUNK the circuit breakers ON the genny may have tripped. check to be sure...don't just look at them.
