Every so often I get a phone about finding lost or misfiled emails. Normally the user has accidentally dragged the email to a folder and misfiled it.
Luckily the search facility in Outlook 2010 is pretty good.
Click on your Inbox and then into the "Search Inbox" field. The toolbar at the top of the screen should now display some search options, the default is to search "Current Folder", change this to "All Subfolders"
99% of the time this will find the email I am looking for. Sometimes the user will want to know what folder the email was misfiled in, simply double click on the email and then click on the File tab. Under the "Move to Folder" option it should display "Current Folder: "