The stale status is indicative of an error state. Essentially it means that the workflow instance is not going to be able to continue to run. One common reason for the stale state is that the payload of the workflow has been deleted or moved. Depending on the version you are running it may also indicate that the Sling events or jobs tied to the workflow instance no longer exist.
Generally if the workflow instance has gone stale it means that the it's dead and you can't restart it so you need to determine if you need to restart the workflow for its payload because it hasn't finished, or just purge it because it's not relevant anymore.