First step: Take a look under the hood: Go Setup > Monitor > Apex Jobs
If you see a DML error, it means that you created a workflow rule that is trying to update more than 1,000 records, or send out more than 1,000 alerts. As are result, you are hitting Salesforce limits and Salesforce is preventing these actions from occurring.
Solution: Deactivate the rule that is trying to execute this mass update. Then recreate a New rule with stricter conditions; ie include date parameters Before X Date and After x Date, so that our workflow rule finders fewer records that qualify to trigger a workflow action. Create additional separate workflow rules to address the other records.