It's not closed. They are doing construction or remodeling or whatever where the open beer bars used to be on the ground floor according to Stickman. Noone seems to know exactly what is going to go in there...could just be remodeled bars, etc...but Nana Plaza is not closed...Here's a link so you can see the pic..it's about midway down the page and below the link is an excerpt from his weekly article...
http://www.stickmanweekly.com/StickmanBangkokWeeklyColumn2010/Sukhumvit-Road-Late-Afternoon-Stroll.htm
"The ground floor of Nana Plaza where the beer bars were recently demolished is not going to stay empty for long. Workers have been laying down concrete and bricks in what appears to be a base for a new structure. The rumour mill is going crazy about what the replacement structure will be. One rumour has it that the new format will a beer garden which will be more open and feature moveable tables and umbrellas, all rather different to how it was. Other rumours say that the same bars will simply be rebuilt! The photo below shows how the ground floor of Nana looked on Friday."