I have a customer where we installed a redundant Ignition setup a few years back. I'm told that they unplugged the PC running the primary Ignition gateway to connect another device for setup. The 2nd gateway picks up as expected. When they plug the primary back in, everything goes red on all of the screens, meaning it isn't getting info from the PLC. They left it plugged in for up to a minute with no improvement. Will things remain red until the two gateways are back in sync (including history)? If so, then we may need to give it more time. The primary has now been offline for a couple of weeks.
You need to monitor the log on the master. Use tail -f ... on the actual wrapper log file if the master gateway web interface isn't starting up. (Perhaps open a support ticket and have IA look over your shoulder.)