Cause:
This issue is caused by one or more index failure.

To identify which index caused a failure, examine the DA Search details.
This can be accomplished by:
1. Go to the Case tab | Case Name in the left panel, then the Searches tab.
2. Click on the Search in question to examine it.
3. At the bottom pane under the Search Details, click on the drop-down menu and select: 2000 failed/error archive indexes.
4. Under the Information column, examine the error message.
5. If there is an index failure for a particular archive, the error message "The search could not be performed due to the index not being searchable.  Index state: Failed" will be reported on the same line as the archive.
6. Make a note of the archive(s) with errors.

Solution:

There are 3 possible methods to resolve this issue:
1. Update (for 32-bit indexes) / Synchronize (for 64-bit index) the failed index volume(s).
2. Restore the failed index folder(s) from a last good known backup.
3. Rebuild the index(s) that corresponding to the archive reporting this error.

Note: If a backup of the failed index prior to its failure is available, it is recommended that the index is restored and updated afterward. This increases the index recovery process quicker since it does not need to rebuild the entire index from a scratch.

Details on how to update, synchronize and rebuild an EV index.
For EV 9.x (32-bit indexes):
1. On the Enterprise Vault (EV) server, open an EV Administration Console (VAC).
2. Expand the Directory on [EV_server] | EVSite | Archives container.
3. Select the appropriate archive container such as Exchange Mailbox or Exchange Journal.
4. Right click on the actual archive name and select Properties.
5. Select the Index Volumes tab.  Right click on the appropriate index volume to 'Update Index Volume' or 'Rebuild Index Volume'.

Note: If the entire index for an archive needs a rebuild, this can be accomplished from the Advanced tab by clicking on the 'Rebuild Index'.

For EV 10.x and higher (32-bit & 64-bit indexes):
1. On the Enterprise Vault (EV) server, open an EV Administration Console (VAC).
2. Expand the Directory on [EV_server] | EVSite | Archives container.
3. Select the appropriate archive container such as Exchange Mailbox or Exchange Journal.
4. Right click on the actual archive name and select Properties | Index Volumes tab.
5. Click on the Manage Indexes. Click on Synchronize and monitor if the index can be returned to health in the quickest manner.  
6. If the problem persists or the or index must be rebuilt, click on the Rebuild rather than Synchronize.

Note: Index rebuild can take a long time depending on its size. Plan appropriately since the index cannot be search while in a rebuilding state.
For more details, refer to the Enterprise Vault Administration guide on how to synchronize or rebuild an index.

To restore the failed index, please refer to the Recovery Section in the Enterprise Vault Administrator's Guide.