The style sheet is a file that basically makes your site look pretty–it’s what controls the layout and the colors and the fonts and all that stuff. It sounds like you somehow downloaded and installed a corrupt version (or at any rate an incomplete version) of BuddyPress that was missing the style.css file.
If you did an automatic install through WordPress, I’d first try just uninstalling and reinstalling BuddyPress and see if that works. Otherwise, you’ll want to look through the files installed on your webhost for a file in your BuddyPress folder called something like style.css (at any rate, it’ll end with .css). If it’s not there, you’d need to add it. I can tell you more about how to do that, if you need, but try the automatic uninstall and reinstall first.
Make sure you activate the default BuddyPress theme or create a suitable child theme. BuddyPress is more than a plugin, you need a compatible theme.
