Sheet metal body part disappear

I need to replace the back panel mounted in the sheet metal box. I suppressed  the old rev and insert the new back panel.  Everything looks fine.  When I reopen the assembly to fix something else, I open that back panel part file from the assembly and I have this on my screen.

The solid body of the sheet metal is gone!  SW only shows me the pem nuts.  What happen here?  Thanks

SolidworksAssemblies