What do you see at the top of the list of messages?
# of conversations selected, options to Mark as Read, or Delete?
Or icons for Search, Edit, and Folders?
If you see the # of Conversations, click the X in the top right corner to close that and get to the magnifying glass and bullet list icons.
Once there, you can click on any message and it will open the message thread in a new window where you can reply.