In different instances, to get your page back may require a particular fix, that can not be generally described here. The style sheet has a lot of items in it, that if coded in the wrong wrong way, could make the page not display. Don't fear though, most edits to the style sheet will not make the page disappear, most that usually happens is that some looks a little screwed up. If you start editing the php code and have no idea what your doing, that could make the page dissapear very easily.
As far as the images in your admin section go, the only thing I can think of that would make the images disappear would be if there was some messing around in the admin section being done. You may need to download the same version of Zen Cart that you are using and upload all the admin section again. I say 'all the admin section' because if some is missing, there may be other parts missing. Some may have just made the buttons disappear using the admin style sheet, or may have deleted the images.
If your still pretty new at Zen Cart and FileZilla, you may want to wait a little bit (to upgrade) if you haven't already upgraded to zencart_1.3.8. Or have someone guide you along in the upgrade via instant message, so it would go fairly quick.