Open Gmail
Click the search bar at the top
Type `in:all` to search all mail, including archived messages
Type the sender, subject, or keyword you want to find
Press Enter
Check the results for messages without the Inbox label
Open the message
Click More
Select Move to Inbox if you want to restore it to the inbox
On mobile, open the Gmail app
Tap the menu icon
Tap All mail
Look for the archived email there