There is a post quick reply button directly under the text area.
The alternative is to use go advanced (next to quick reply) and write your answer there and press submit reply.
I suppose you have used the reply to thread green button at the top of the page (next to post new thread, I don't know the purpose it serves) which reloads the page and you lost the text.
Note that you can always use back (of your browser) in advanced mode and the message will still be there but in the quick reply it is lost.
Alex