When you delete a repository from the Nexus UI, nexus will remove the repository from any groups it belongs too and move the repository contents to it's trash folder. Sometimes you will want to restore this repository back into service.
To restore a deleted repository with id of 'releases':
1. mv sonatype-work/nexus/trash/releases sonatype-work/storage so that you end up with sonatype-work/storage/releases
2. Recreate the repository with the same repository id 'releases' using the repositories tab user interface.
3. Add the repository to any groups that it was in before.
The act of creating the repository in the UI will force a reindex of the previous repository storage contents.