Why window alert cannot work in any browser

I use window.alert() in my function previously work, but now it cannot work properly.

showmessage("hi")

Protected Sub ShowMessage(ByVal stringMessageContent As String) If (stringMessageContent.Trim().Length > 0) Then

        stringMessageContent = stringMessageContent.Replace("\r\n", " ")
        stringMessageContent = stringMessageContent.Replace("'", "")
        ClientScript.RegisterStartupScript(Me.GetType(), "ClientScript", "<script language='JavaScript'>  window.alert('" + stringMessageContent + "');</script>")

    End If


End Sub

But the message not show in any browser. pls give me a solution ASAP