Slicers can be lost from the "Slicers Horizontal" and "Slicers Vertical" sections of the PowerPivot Field List in Excel.
Changing the values selected in the slicer often still updates the PivotTable / Chart though it is no longer possible to manage the slicers using the PowerPivot Field List.
The attached files are a full repro for the issue, including video. The Model.xlsx file is the broken one (i.e. that results from following the steps shown in the video). The Model_working.xlsx file is the one that is used from the start of the video (i.e. before the issue occurs).
I have managed to replicate this issue on the PP2012 CU2 update.