These are supposed to be the steps followed to troubleshoot CoreOS in general.
ssh into the machine and check for initial login warnings.
systemctl status, this will list out the status of different services.
systemctl list-units, this will list the services with their status
systemctl status name.service, to find out the status of individual services.
journalctl -u name.service, to list out the logs of service.
You can use various journalctl parameter combination to fetch the logs of different time.
sudo systemctl start application.service