The program enters a loop that lasts until it gets a file name. Inside the loop, the program uses Dir$ to look for a file in A:\. If the program does not get an error, it sets got_file to True to end its loop.

If the Dir$ function raises an error, the program tells the user there is a problem and asks what to do. If the user picks Abort, the program displays a message and exits the subroutine. If the user clicks Retry, then the code continues its loop to try again. If the user clicks Ignore, then the program fills in a default blank file name and sets got_file to True so it exits its loop.

After the loop ends, the program checks the file name to see if it found a file or the user clicked Ignore. If the user clicke Ignore, the program takes some default action. If the program found a file, it lists the files in A:\.

Private Sub cmdList_Click()
Dim got_file As Boolean
Dim file_name As String
Dim had_error As Boolean
Dim txt As String

    got_file = False
    Do Until got_file
        ' Get a file in A:\.
        On Error Resume Next
        file_name = Dir$("A:\*.*")
        got_file = (Err.Number = 0)
        On Error GoTo 0

        ' See if we got a file name.
        If Not got_file Then
            ' Trouble. The drive may be empty.
            ' Ask the user what to do.
            Select Case MsgBox("Error reading A drive. " & _
                    Err.Description & vbCrLf & "Try " & _
                        "again?", _
                    vbAbortRetryIgnore, "Error Reading " & _
                        "Drive")
                Case vbAbort
                    ' Cancel the operation.
                    MsgBox "Operation canceled"
                    Exit Sub
                Case vbRetry
                    ' Continue the Do loop to try again.
                Case vbIgnore
                    ' Take a default action.
                    file_name = ""
                    got_file = True
            End Select
        End If
    Loop

    ' See if the user picked Ignore.
    If Len(file_name) = 0 Then
        ' Ignore. Take a default action.
        txt = "Here's the default text."
    Else
        ' Success. List the files in A:\.
        txt = "Files:" & vbCrLf
        Do While Len(file_name) > 0
            txt = txt & "        " & file_name & vbCrLf
            file_name = Dir$()
        Loop
    End If

    MsgBox txt, vbOKOnly, "Results"
End Sub