Sometimes this issue occurs when restarting Nexus [my version is 2.0.5].

The following steps to clean Nexus mostly help to get it up and running again

# Clear archetype-metadata.xml
– Goto /sonatype-work/nexus/storage
– perform the following command “rm **/*/archetype-metadata.xml

# Clear repository-metadata.xml
– Goto /sonatype-work/nexus/storage
– perform the following command “rm **/*/repository-metadata.xml

# Clean .nexus/attributes directories with size 0
– Goto /sonatype-work/nexus/storage
– perform the following command “find . -type f -size 0 |xargs rm”

After this, restart the service via ” sudo service nexus restart”