If the index has been marked as failed you will not be able to search the particular associated vault.....run the indexvolumereplay tool and "update" that failed index. Just because it is marked as failed does not necessarily mean it is corrupt - but without seeing logs and avtrace, I couldn't really tell you why it got marked as failed.
When you do the update on the failed index (update and not rebuild) - check the logs and also refresh the tool gui to see if it fails again right away or if it finished the update successfully. If it is updated successfully you can search again but if it fails again it may be an idea to log a support call so we can determine why it is failing.