Resolution:  

1. Make sure that the box is checked on the General tab of the Timesheet Approval Workflow for Allow Employees to Reopen Closed Approval Records and there is at least one employee in the field beside it.  
2. If the last step of the Timesheet Approval Workflow is to approve Timesheet Lines, have one of those approvers go into the Approval Center.  
3. In the Approval Center, select Timesheet Lines and check the box for Show all Pending and Completed  
4. Click the Action Required field and select Reopen for the line item(s) in question.  
5. Enter a Comment which is required when reopening a timesheet.
6. Click Reopen.  
7. If the last step of the Timesheet Approval Workflow is to approve the overall Timesheet, the Timesheet can be reopened in the Approval Center or it is also possible to Reopen the timesheet by selecting it in My Stuff > Timesheet and selecting the Timesheet in question. Then click Reopen. A comment is required here as well.  
8. Once a timesheet has been reopened, the timesheet can be edited or rejected.

Note: Posted Timesheets cannot be reopened.

Knowledge Base Reference Number: KBA #97967