The PowerPivot com add-in can occasionally disappear from the ribbon. Follow the directions below to force it to appear.
- Go to File > Options
- Go to Add-Ins, then at the bottom of the page, select COM Add-ins and click Go.
- In the COM Add-Ins window, deselect the check box for Microsoft Office PowerPivot for Excel 2013 COM Add-in, then click OK.
- Go back to the COM Add-ins window (following steps 1 and 2 above) and select the check box for Microsoft Office PowerPivot for Excel 2013 COM Add-in.
- The PowerPivot ribbon should reappear.