Many times we might have noticed an email notification saying, "A scheduled workflow failed to run. Check the workflow is not already running or in an error state". I recently faced this issue and my mail box was flooded with emails notifications.
And there is no much information in the email except it is pointing to the workflow's schedule page and to workflow's status page.
When checked ULS logs and found an exception, "Microsoft.SharePoint.SPException: The sepcified user or domain group was not found".
Then I check for the user who scheduled the workflow and that user was no more working and his id got removed.
Fix: Open all schedule workflows's schedule and click "Save" with different user.
Best practice: It is best practice to publish or schedule the workflows with a service account on PROD environment.
Other details: There could be other reasons to fail the scheduled workflows, below are the some helpful links to troubleshoot.