what could be the possible reasons for NameNode going down
That's way too general a question to provide a useful response.
Give us some background like:
1.Overview of the cluster config including CDH/HDP version, if HA is enabled
2. Any recent changes
3. Errors/Exceptions in the NN log files
The only point of failure in Hadoop v1 is NameNode. If NameNode fails, the entire Hadoop cluster will fail. Actually, there will be no data loss, only the cluster job will be shut down because NameNode is just the point of contact for all DataNodes and if the NameNode fails then all communication will stop.
In my experience, lack of computer resources was the biggest cause.
It is always necessary to prepare disks and memory with sufficient space.