not all my incoming mail goes to the inbox

Just bought Mac Mini with Yosemite last week. Mail was working well on it until today. Incoming mail is showing up attached to the Sent message in the Sent Mail Box, if it is a response. Other message are opening in the Inbox of Mail on my Mac Book Pro, but not on the Mac Mini. Ws working well up until now. Is there a Setting that has become unglued?