You need to do something like this:
stop-yarn.shin the 2.x serie)
rm -Rf /app/tmp/hadoop-your-username/*
bin/hadoop namenode -format(or
hdfsin the 2.x serie)
the solution was taken from: http://pages.cs.brandeis.edu/~cs147a/lab/hadoop-troubleshooting/. Basically it consists in restarting from scratch, so make sure you won't loose data by formating the hdfs.
~ Answered on 2012-08-12 17:29:07