运行时，出现出错提示,“实时错误91,对象变量或with或块变量未设置”，如何解决？

编写一高考查询系统，运行时出现如标题所示错误，请问各位高手,如何解决？谢谢

Private Sub Form_Load()

ado.ConnectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + App.Path + "\db1.mdb"

ado.CommandType = adCmdText

ado.RecordSource = "select * from db1"

ado.Refresh ‘运行至此，出现出错提示“Refresh 的方法 'IAdodc ’失败"

End Sub

Private Sub Label2_Click()

_ado.Recordset.Close ‘运行至此，出现出错提示,“实时错误91,对象变量或with或块变量未设置”，
_ ado.Recordset.Open "select * from db1 where 准考证号= ' " + Txt1 + " '"

If ado.Recordset.EOF Then
MsgBox "数据库中没有你想查询的", 16, "错误"
Else

Form2.Txt1.Text = ado.Recordset.Fields("姓名").Value
Form2.Txt2.Text = ado.Recordset.Fields("性别").Value
Form2.Txt3.Text = ado.Recordset.Fields("语文").Value
Form2.Txt4.Text = ado.Recordset.Fields("数学").Value
Form2.Txt5.Text = ado.Recordset.Fields("外语ï").Value
Form2.Txt6.Text = ado.Recordset.Fields("综合Ï").Value
Form2.Txt7.Text = ado.Recordset.Fields("总分").Value
Form2.Txt8.Text = ado.Recordset.Fields("录取学校").Value
Form2.Show
Form2.Caption = RTrim(ado.Recordset.Fields("姓名").Value) & "的成绩"

End If

End Sub