Pretty simple, just what it says. FoW was active on the page, map was blacked out for players, but revealing did nothing. Turning FoW on/off also had no impact, so we were stuck with a black screen. Tried this in one game as a player, another as GM. All of us are on base accounts, so no dynamic lightning settings involved. Also had a few problems with tokens not updating, but those seemed to resolve after exiting Roll20 and reopening.