Simple Db Update Question

Hi folks. I'm trying to update a single field in a database table with the input from a single text box, but I'm getting "Insert Error: Column name or number of supplied values does not match table definition". I know at least part of the problem is that I haven't specified the name of the table field, but I'm not sure where to do this. The table in question has many fields, but I only want to update one. The submit button is "sendMail", which calls UpdateDB.

I'm learning and trying to limit myself to 1 idiotic question per week. Please have mercy. Thanks. Chris
private void UpdateDB(string cmd)
{
// point to connection string in web.config
string connectionString = "Data Source=localhost;Initial Catalog=databaseName;Integrated Security=True";
// create connection object; initialize; open
System.Data.SqlClient.SqlConnection connection = new System.Data.SqlClient.SqlConnection(connectionString);
try
{
connection.Open();
// create sqlcommand object and assign connection
System.Data.SqlClient.SqlCommand command = new System.Data.SqlClient.SqlCommand();
command.Connection = connection;
command.CommandText = cmd;
command.ExecuteNonQuery();
}
finally
{
connection.Close();
}
}

protected void sendMail(object sender, EventArgs e)
{
string cmd = @"INSERT INTO tableName VALUES ('" + this.TextBox2.Text + "')";
UpdateDB(cmd);
// send smtp mail
...
}