It’s likely that you removed a div
from one of your PHP files (like sidebar.php).
There’s no “reset this theme” capability built into WordPress, but if you can get a new copy of the theme, you could compare those PHP files with the one you see under Design>Theme Editor. Then you’ll see what’s causing the problem.
Even easier would just be to re-upload the theme over top of the one you edited wrong. That would get rid of all the changes you made (good or bad), but it’d look and work like new.