Had this same exact issue on the old forums. Reply to a post and click Post. For whatever reason the site generates a site error. All your reply is gone meaning you either need to remember to copy the response before clicking Post or you need to type it all in again. It happens a lot and for no reason. For example I have successfully replied to 2 posts but the third reply fails.
Fix the server errors so this doesn't happen. It happens a lot right now.
Allow some way to get back to the reply we just typed when this happens so we can copy it and try again.