Well, I'm not well-versed in DNN 3.x, but in 4.x the code you're looking for is in USERS.ASCX.VB in the Page_Load setion -- see highlighted portion of DNN 4.x code below. Note: Do NOT use this code in your DNN 3.x instance! I'm posting it for illustration purposes only.
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Try
'Add an Action Event Handler to the Skin
AddActionHandler(AddressOf ModuleAction_Click)
If Not Page.IsPostBack Then
'Load the Search Combo
ddlSearchType.Items.Add(AddSearchItem("Username"))
ddlSearchType.Items.Add(AddSearchItem("Email"))
Dim profileProperties As ProfilePropertyDefinitionCollection = ProfileController.GetPropertyDefinitionsByPortal(PortalId, False)
For Each definition As ProfilePropertyDefinition In profileProperties
ddlSearchType.Items.Add(AddSearchItem(definition.PropertyName))
Next
'Localize the Headers
Localization.LocalizeDataGrid(grdUsers, Me.LocalResourceFile)
BindData(Filter, ddlSearchType.SelectedItem.Value)
End If
Catch exc As Exception 'Module failed to load
ProcessModuleLoadException(Me, exc)
End Try
End Sub
Note that the "Username" and "email" items are added individually while the other searchable profile field names are loaded from the database. Unless the previous person managing your DNN instance edited the USERS.ASCX.VB file, I doubt your problem is with the USERS code.
VERIFY PROFILE PROPERTIES HAVE NOT BEEN DELETED
At this point I'd recommend making sure your site still HAS profile properties.
Method 1: Log in as ADMIN or HOST, going to ADMIN->USER ACCOUNTS and then select "MANAGE PROFILE PROPERTIES". Verify that all the expected profile fields are still there. You SHOULD see the default user profile fields for name info (prefix, firstname, middlename, lastname, suffix), address info (unit, street, city, region, country, postalcode), contact info (telephone, cell, fax, website, IM) and preferences (timezone, biography, preferredlocale). It's possible that someone deleted all the fields (I've seen it happen more than once) not realizing that deleting them really is deleting them and not just hiding them from the user.
Method 2 (SQL): If you know your portal ID number (it will be '0' if you only have one portal), go to HOST->SQL, copy-and-paste the following query into the box and click "Execute":
exec GetPropertyDefinitionsByPortal {your portal ID number here}
Look in the column "DELETED" and make sure all values are set to "FALSE".
Also -- if you have not already done so, undo you previous edit(s) to the USERS.ASCX file to avoid having your changes introduce new issues.
Check into whether your profile properties have been deleted and post what you find-
-mamlin