There are always ways to bring a system back. I will repeat, literally, what I just said:
Time for you to start digging! You’ll need to figure out how your infrastructure is built up… Which Elastic server(s) does your Graylog expect to talk to? Where are they located? Why is Elastic “missing”? Who changed something, when why where and how? These things usually don’t go tits-up without reason.
You will need to understand the design and configuration of your environment. You’ll need to understand the parts that make up the Graylog stack and what the respective parts do. You’ll also need to figure out how to troubleshoot the individual components, because Graylog proper is the only product supported on these forums. MongoDB and ElasticSearch are products from different companies; we can help you with them, but not support them.