This is common.  Go to the material room, select the item that is invisible, then move look in the poser surface window for the transparency dial.  Move the dial value back and forth and the item should reappear.  Put the value back to what it was.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi