Cryptographic exception Was unhandled

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C#

.NET

C#4.0

.NET4

When I am trying to dercypt my encrypted text I am getting the following error
"Bad Data:Cryptographic exception Was unhandled"

I am getting the error at following line of code

byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);

this is my code for decrption

public string decrypt(string code, string Key)
       {
           bool useHashing = true;
           byte[] keyArray;
           byte[] toEncryptArray = Convert.FromBase64String(code);

           //System.Configuration.AppSettingsReader settingsReader = new AppSettingsReader();
           //Get your key from config file to open the lock!
           //string key = (string)settingsReader.GetValue("SecurityKey", typeof(String));
           string key = Key;
           if (useHashing)
           {
               MD5CryptoServiceProvider hashmd5 = new MD5CryptoServiceProvider();
               keyArray = hashmd5.ComputeHash(UTF8Encoding.UTF8.GetBytes(key));
               hashmd5.Clear();
           }
           else
               keyArray = UTF8Encoding.UTF8.GetBytes(key);

           TripleDESCryptoServiceProvider tdes = new TripleDESCryptoServiceProvider();
           tdes.Key = keyArray;
           tdes.Mode = CipherMode.ECB;
           tdes.Padding = PaddingMode.PKCS7;

           ICryptoTransform cTransform = tdes.CreateDecryptor();
           byte[] resultArray = cTransform.TransformFinalBlock(toEncryptArray, 0, toEncryptArray.Length);

           tdes.Clear();
           return UTF8Encoding.UTF8.GetString(resultArray);

       }

I am not getting what is the probles as this code was working properly before.plz help. thanks in advance
swati

[edit]Code block added - OriginalGriff[/edit]