I used a pair of tweezers to pry under the rim and lift it a bit until I could insert straight edge screwdriver to lift the pop up. This was in a japanese soaking tub that was full of water and I did not want to have to bail or siphon it empty. But now the tub is dry I can use these methods to get it back up again and fix the problem, thank you!
I used a 1.25" suction cup off Amazon (cup hook) to get stopper to pop up. If all else fails, go under the sink, undo P trap & use a long screwdriver to push up stopoer.