dave is right ..re-import the low level mesh from Zbrush back into Maya and also make sure you uncheck Create Multiple Objects or you may have problems with vertex reordering.
There are several work flows you can use from Zbrush, you can make the normal map in Zbrush and export it or you can export both the high and low level models and use Transfer Maps in Maya to create a normal map, alternatively you can use both the high and low obj's with Xnornal (separate app) to create them.