I had made some changes in Gmail account due to which, if I reply to or forward some mail then it deletes the basis mail from my inbox automatically. If I want it back in inbox then I need to search the mail with the subject and then mark it in inbox then the same mail comes back to the inbox.
Make sure you aren't clicking "send and archive" when sending a reply. The archive command strips the inbox label from the mail. If that message/conversation didn't already have a label then the only way to find it is by searching all mail.
To hide the "send and archive" button then go to:
Settings/general/ "Hide "Send & Archive" button in reply" section