http://tribesftw.net/sp00f/screenies/11202011_13_13.jpg
Saved this .zbr last night, loading it now but the edit is greyed out. I don’t get it- why does the edit button go grey and never come back again in some instances?
Use File>Save As to save as a “Project” file, rather than Document>Save As.
Saving the Document (a .ZBR file) only saves a 2.5D (2D plus depth info) version of the canvas. As halogenic says, to ensure that a 3D model is retained you must ether save the Prject (File>Save As - this saves everything) or the selected model (Tool>Save As).
Check the ZBrush documentation (in the Documentation folder of your ZBrush installation) so that you can get to grips with the essentials before embarking on anything important. 
For sure thanks, I’m tryin’!
This was helpful for me too. I was searching why my document came up un-editable after saving and came upon this thread.
As for the documentation, sometimes it isn’t super clear especially if you are new to this kind of thing. I always look there first there and google as well but its still nice to have a forum to come to