I am little confused. Are you saying you put it in the tablet upside down or did you put it in an actual holder (not inside the tablet) upside down?
I opened the SIM holder by inserting a paper clip.The holder came out, I placed the sim facing down. I pressed it back, opened the phone but it says there is no sim. I downloaded a Lumia 920 manual (I bought the Nokia Lumia brabd new with no Manual) and only then did I find out that the sim should be inserted facing up.
My problem now is I can't get the sim holder to open so I can invert the sim.