It happens once in a while. When you are performing an operating with HDFS like adding new data you may see this message:

WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
put: Cannot create directory /users/txt6. Name node is in safe mode.

Safemode in HDFS can be switched off with one command:

hdfs dfsadmin -safemode leave

Once you’re run that command you will be able put data in to HDFS again.