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();

user_name  是字符串类型在赋值的时候要加单引号

不过这种写法我在好多书上见过，也不清楚为什么
试试这个"insert into Users(user_Question,user_Answer) values(?,?) where user_name='"+txtName.Text.Trim()"'";
或者你把sql语句先定义一下，例如：string sql＝"insert into Users(user_Question,user_Answer) values(?,?) where user_name="+txtName.Text.Trim();
然后在com＝new OleDbCommand(sql,con)

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,

com = new OleDbCommand("insert into Users(user_Question,user_Answer) values(?,?) where user_name='"+txtName.Text.Trim()+"'",con);

5楼的说得对，做技术的要严谨，该要单引号的要要，不该要的就不能要！

顶一下五楼