Go to your Settings > System Jobs. Filter this view by Type = Workflow and View = Suspended System Jobs. Open one of the instances up, there may be more information in there as to why it failed. Sometimes it could be due to a mistake in the design of the workflow, or because a referenced record has been deleted or the user doesn't have permissions on a referenced record, etc.
Matt Wittemann, MVP for CRM (http://icu-mscrm.blogspot.com)