How did I break the Path Tracer?

Working on materials for a truck. Was toggling from Lit to Path Tracer to see the difference it makes to materials. Was looking at just a side of the truck. Then the materials went black, I zoomed out and the rest of the truck was fine (overall scene does look different, low contrast and shadows are off) Lit is fine but if I switch to Path Tracer, that side is broken. Have restarted computer, deleted some cache files, rebuilt things, applied different materials, nothing seems to work. How can I break the Path Tracer?