ASP.Net - Base64 convert HTML Turkish character problem

TURKISH_History_Html row data type varbinary. It saves the html code to the row in byte-64 data type as in the photo below. But Turkish characters are not displayed. Turkish characters ?? Displayed as. How can I show Turkish characters?

string theString2 = Encoding.UTF8.GetString(degerler.TURKISH_History_Html);

                    string base64Encoded2 = (theString2);
                    string base64Decoded2;
                    byte[] data2 = System.Convert.FromBase64String(base64Encoded2);

                    base64Decoded2 = System.Text.ASCIIEncoding.ASCII.GetString(data2);