DIAGNOSIS That solid red light means the node has lost its connection to the main router or the mesh network. It's essentially stranded and can't talk to the rest of the system.
FIX First, don't panic. Unplug that red-light node from power. Leave it unplugged for a full minute. While you wait, go check your main router node—make sure it's still on and has a solid blue light. After the minute, plug the problem node back in and move it much closer to the main router, ideally in the same room. Give it a few minutes to boot up and try to re-establish the connection. The light should cycle and eventually turn blue.
IF THAT DIDN'T WORK You might need to force a re-pair. On the problem node, find the reset button (it's a tiny pinhole). With the node powered on, press and hold that button with a paperclip for about 10 seconds until the light flashes amber. Then open the Linksys app, go to Mesh Network, and follow the prompts to add a new node. It will guide you through syncing them again.
HEADS UP After you get it working and move it back to its final spot, if the red light comes back, that location is just too far or has too many walls for a stable wireless backhaul. You might need to find a middle-ground spot or consider using a wired Ethernet backhaul if possible.