I think it would be okay to create a 'Site new version coming very soon' landing page and take the main site down as you work on it. But main and the most difficult target would be letting users know what your site exactly does, how easy it is, how different it is: all of them without showing the actual site! ( maybe some simple text on landing page, and mostly through well drafted text publicity on social networks ).
Show users before adding the functionality that they actually need it.( that it was inherent). If you are able to do this correctly, then you could get yourself a good fanbase.
Basically, I would suggest to improve on promotional strategies before starting coding again (after a landing page). And also keep a clear view on how are you going to generate income( if you want to call yourself a startup :) )