The ethernet ports suddenly stopped working on airport extreme wondering why and how to fix the problem

I use the ethernet cable to connect my airport extreme to a linksys router to extend the network but suddenly all the ethernet cables on airport extreme stopped working and now the linksys won't get a connection. I was wondering why this might of happened and if there is a solution to fix the problem. Also i wondering if i should get rid of the linksys and get and airport express to just extend the network.