I don't know what I did in my model but when I reload it, I see that the part is invisible.
When I hover over the functions of the model tree, I can see their edges highlighted in the viewport, but the model itself remains invisible. As if it were made of a perfectly transparent material.
If I export it to STEP for example, the part is quite correct.
+1 strong changes that the body is hidden, just look in the body/volume tree if your cube is white and not yellow or blue if it is hidden.
Note that in the 2018 the TAB key hides the parts in the assemblies, possible that this is also the case with the bodies in a part, to be tested. (key that proves to be practical)