public static string constring = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=Data.mdb";

con = new OleDbConnection(constring);
con.Open();
com = new OleDbCommand("insert into Users(user_Question,user_Answer) values(?,?) where user_name="+txtName.Text.Trim(),con);
com.Parameters.Add(new OleDbParameter("question",OleDbType.VarChar,50));
com.Parameters.Add(new OleDbParameter("answer",OleDbType.VarChar,50));
com.Parameters["question"].Value = cbQuestion.SelectedItem.ToString();
com.Parameters["answer"].Value = txtAnswer.Text.Trim();
com.ExecuteNonQuery();
con.Close();

还是不行,还是说SQL语句没有;结尾,
是不是在一个FORM里面用了
OleDbDataReader dr = com.ExecuteReader();以后就不能再用
com.ExecuteNonQuery();了,