I'm with gmail for my sins (along with another for security) and this is what I got from Google.
Click on All Mail on the left. When you see the message, open it, and choose the Move to Inbox option to "un-archive" it. Alternatively, you can use the search feature to locate the message (the search box should be towards the top of the Gmail page). An archived email is just an email with the inbox label removed.
Truth is like a threshing machine; tender sensibilities must keep out of the way.
Herman Melville
