The simplest way to find an archived email
is to use Gmail's search function.
On the top of the Gmail page,
type in the email address of the sender
or a few words from the email's subject
or body and hit return.
Gmail will find and display emails,
whether they are in your inbox, sent messages,
archived, and so forth.
You can also use the "all mail" option
to show all of your emails at the same time.
This includes new emails, those in the trash,
sent messages, and, of course, archived email.
To show all of your emails,
click on the "More" option located in the column
to the left of your email or email list,
then click "All Mail" and scroll through
your entire Gmail trove, including archived emails.
To unarchive any archived email,
select the "Move to Inbox" option,
located in the toolbar above the archived message.