Hi Sharon,
Welcome to RootsChat
The blue "Reply" button that you pressed to reply to an old topic is the correct button to press to reply to any of the topics on RootsChat. There are some topics that have a little padlock these you are unable to reply as they are "locked" butcan be unlocked if you contact a moderator.
The blue reply buttons appear at the top and bottom of every topic, and to start a new topic there is the blue "New Topic" button.
If you can advise which topic you are wishing to reply to I can check this out for you.
Regards
Sarah