Namenode not starting because of Namenode not formatted error

[#1]

I cannot format name node since it is my production cluster. Namenode is not starting because of this error though:

Please suggest what needs to be done to fix this safely

org.apache.hadoop.hdfs.server.namenode.FSNamesystem

Encountered exception loading fsimage java.io.IOException: NameNode is not formatted. at org.apache.hadoop.hdfs.server.namenode.FSImage.recoverTransitionRead(FSImage.java:222) at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.loadFSImage(FSNamesystem.java:1097) at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.loadFromDisk(FSNamesystem.java:779) at org.apache.hadoop.hdfs.server.namenode.NameNode.loadNamesystem(NameNode.java:614) at org.apache.hadoop.hdfs.server.namenode.NameNode.initialize(NameNode.java:676) at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:844) at org.apache.hadoop.hdfs.server.namenode.NameNode.<init>(NameNode.java:823) at org.apache.hadoop.hdfs.server.namenode.NameNode.createNameNode(NameNode.java:1547) at org.apache.hadoop.hdfs.server.namenode.NameNode.main(NameNode.java:1615)

[#2]

@Simran kaur, we don't usually format a production cluster unless it is absolutely necessary? Can you elaborate on what happened to the namenode (like hdfs command that had been executed) before the NN fail to start?