i just spent a couple of hours going jelly-brain, but i'm happy to say it's fixed, so i thought i'd share, incase others have the same problem.
i could not see the themes in the 'presentation' admin page. not the default two themes or any new uploaded themes. i eventually found (from a post here) about the permissions issue, so i went around 777'ing lots of files, but that didn't help. eventually i figured out that you can set permissions for files aswell (maybe i'm stoopid, but i didn't make this idea-leap at first) so i 777'd the wp_content file itself (and enclosed items for good measure) and now i can see themes. yippeeee!