It is probably best to use revision manager... but you can also fix lost part references by going to Edit...Links.
The dialog box is laid out a little irritatingly but if you drag the Source file column wider you should be able to find the entry for your missing part. Select it, click change...
I think I know what you want to do...
Select draw in view, then sketch out the closed profile outline for your hatched area.
Don't attempt to fill the area by simply clicking for the hatch in the area you want - instead drag and select the outline you've just drawn first *then* click on...