comment boxes are by default only shown on single posts and static pages;that on the blog, I haven't got an option for comments so no-one can reply
I don't know, why comments are (now) closed on your single posts after the change of theme,
but you can control that from:
- dashboard - settings - discussion (will only have effect for posts published after the changes)
and for each individual post:
- dashboard - posts - all posts - click quickedit below the post titles - tick 'allow comments'