How to fix single face mesh? The model in Blender shows the mesh double faced and good looking but when exported into game looks like this. How can I fix it?
It looks to me as though you have backface culling enabled in your game engine.  Are you using blender game engine?

You can either add a solidify modifier as gandalf3 said in the comments.  Or, if this is the only direction you will be viewing the curtains from you can flip the normals on the curtains.