There are 0 datanode(s) running and no node(s) are excluded in this operation

Two things worked for me,

STEP 1 : stop hadoop and clean temp files from hduser

sudo rm -R /tmp/*

also, you may need to delete and recreate /app/hadoop/tmp (mostly when I change hadoop version from 2.2.0 to 2.7.0)

sudo rm -r /app/hadoop/tmp
sudo mkdir -p /app/hadoop/tmp
sudo chown hduser:hadoop /app/hadoop/tmp
sudo chmod 750 /app/hadoop/tmp

STEP 2: format namenode

hdfs namenode -format

Now, I can see DataNode

hduser@prayagupd:~$ jps
19135 NameNode
20497 Jps
19477 DataNode
20447 NodeManager
19902 SecondaryNameNode
20106 ResourceManager

I had the same problem after improper shutdown of the node. Also checked in the UI the datanode is not listed.

Now it's working after deleting the files from datanode folder and restarting services.

stop-all.sh

rm -rf /usr/local/hadoop_store/hdfs/datanode/*

start-all.sh

@Learner,
I had this problem of datanodes not shown in the Namenode's web UI. Solved it by these steps in Hadoop 2.4.1.

do this for all the nodes (master and slaves)

1. remove all temporary files ( by default in /tmp) - sudo rm -R /tmp/*.
2. Now try connecting to all nodes through ssh by using ssh username@host and add keys in your master using ssh-copy-id -i ~/.ssh/id_rsa.pub username@host to give unrestricted access of slaves to the master (not doing so might be the problem for refusing connections).
3. Format the namenode using hadoop namenode -format and try restarting the daemons.

On my situation, firewalld service was running. It was on default configuration. And it don't allow the communication between nodes. My hadoop cluster was a test cluster. Because of this, I stopped the service. If your servers are in production, you should allow hadoop ports on firewalld, instead of

service firewalld stop
chkconfig firewalld off

I had same error. I had not permission to hdfs file system. So I give permission to my user:

chmod 777 /usr/local/hadoop_store/hdfs/namenode
chmod 777 /usr/local/hadoop_store/hdfs/datanode