We have worked several cases where BISM icons are no longer present in a SharePoint Library. In the below screenshot you can see the icon is a blank square.
We have found that the mapping is no longer present in the DOCICON located in one of the two directories (dependant on the version you are using):
C:\Program Files\Common Files\microsoft shared\Web Server Extensions\14\TEMPLATE\XML
C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\XML
The missing mapping is: <Mapping Key="bism" Value="PowerPivot\icbism.png" />
This points to the icbism.png
You will want to add this to the DOCICON and Save.
Note: You may need to Stop and Start IIS, because the file may have a lock on it.
As you can now see the BISM Icon is now present!