Online poker technical errors abound, and sometimes they are very strange indeed.
Tonight while playing on a free poker site operated by Zen Entertainment for Hustler Casino near Los Angeles, I was in a tournament where my hole cards and the board cards were "invisible" to me. When I was in the blinds -- and not in the blinds -- no cards appeared on my computer screen, and the cards that appeared on the "board" appeared as "black" -- see the photos below.
I reported this to support at Zen and I will let you know what happened. When the problem did not correct itself, I logged out. Then I logged back in to go to another game and found that the "computer" still had me as active in the game I left. So I went back to the game and found that the problem had fixed.
Still, I had lost nearly one-fifth of my starting stack.