The System.IO namespace contains the StreamReader class allow reading of files.
You can then read the entire contents of the text file into a string, by using ReadToEnd() method.
Remmber to close the StreamReader using the Close() method when finished.
<!DOCTYPE html>
<script runat="server">
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim sr As New System.IO.StreamReader(Server.MapPath("test.txt"))
Dim sContent As String = sr.ReadToEnd
sr.Close()
Response.Write(sContent)
End Sub
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Button ID="Button1" runat="server" Text="Button" />
</div>
</form>
</body>
</html>