Your original model doesnāt go anywhere, you should be able to select it via the available tools in the tool menu, as it is a separate tool from the retopology zsphere tool. To start the retopo process, you would have had to create a new Zsphere tool. You can select this again by selecting the sphere tool after you go back to your original model.
If , however, you alter you original mesh after you have already started a zsphere retopo tool, the changes will not reflect in the existing retopo project, and you will have to start a new one to incorporate those changes.
You can save your progress on the retopo zsphere by de-selecting āedit topologyā, then saving that tool. It is not strictly necessary, it will still be there in the same Zbrush session when you can switch between tools freely, but better safe than sorry.
[note] this info is based on Zbrush 3.1, I suppose it could have changed in the new version.