Can't send messages

Today I received 2 messages. It seems that I cannot reply to the,. There is the "reply" button, but nothing happens. Tried entering the guy's screen name to get the profile (so I could sent him a message). It says that the user cannot be found (or some such). Under "Communication" There is Inbox and Sent - but no option to "write" or "send". How do reply to the guys!