Option Explicit
Public wbA As Workbook

Private Sub CommandButton2_Click()
If Me.TextBox1 = "" Then
MsgBox "Выберете файл!"
Else
If IsBookOpen(Me.TextBox1.Value) Then
MsgBox "Файл уже открыт!"
Exit Sub
Else
MsgBox "Будет открыт указанный файл"
Set wbA = Excel.Workbooks.Open(Me.TextBox1.Value)
End If
End If
End Sub

Private Sub CommandButton3_Click()
If wbA Is Nothing Then
MsgBox "Книга уже закрыта!"
Else
If MsgBox("Книга будет закрыта!", vbOKCancel) = 2 Then
    Exit Sub
    Else
If MsgBox("Сохранить изменения в " + wbA.Name + "?", vbOKCancel) = 2 Then
wbA.Close (False)
Else
wbA.Save
wbA.Close (False)
End If
End If
End If
End Sub

Sub tmr1()
If userform1.wbA Is Nothing Then
userform1.Label1.ForeColor = &HFF&
userform1.Label2.ForeColor = &HFF&
userform1.Label1.Caption = "O"                         'символьный шрифт - крестик
userform1.Label2.Caption = "Файл не открыт"
Else
userform1.Label1.ForeColor = &H8000&
userform1.Label2.ForeColor = &H8000&
userform1.Label1.Caption = "P"                         'символьный шрифт - галочка
userform1.Label2.Caption = "Файл открыт"
End If
End Sub

Function IsBookOpen(wbName As String) As Boolean
    Dim wbBook As Workbook: On Error Resume Next
    Dim intPos%, filename As String
      intPos = InStrRev(wbName, "\")
      filename = Right(wbName, Len(wbName) - intPos)
      wbName = filename
    Set wbBook = Workbooks(wbName)
    IsBookOpen = Not wbBook Is Nothing
End Function

userform1.wbA Is Nothing