Error Ado.Net/ Sql/C# need help please

First off - You should be concerned about your application's security. Appending the TextBox value directly into your SQL query is a BIG "no no" because that can lead you to SQL Injection attack , use should Parameterized query instead for security and more cleaner codes. 

Second - Since you are only dealing with a single table from your database then you should use DataTable and not DataSet.

Here's a quick example:

protected void BindGrid(){
    DataTable dt = new DataTable();
    using(SqlConnection connection = new SqlConnection("YOUR CONNECTION STRING HERE")){
        string sql = "Select * from tblStudent where ID = @ID";
               
        using(SqlCommand cmd = new SqlCommand(sql,connection)){
                connection.Open();
cmd.Parameters.AddWithValue("@ID", Convert.ToInt32(txtID.Text));
                using(SqlDataAdapter ad = new SqlDataAdapter(cmd)){;
                ad.Fill(dt);
//check if the query returns any data
                if (dt.Rows.Count > 0) { 
                       GridView1.DataSource = dt;
                       GridView1.DataBind();
                }
                else{
                     //No records found
                }
                }
        }
    }
}


protected void Page_Load(object sender, EventArgs e){
        if(!IsPostback)
             BindGrid();
}

string strQuery = "Select * from tblStudent where ID = " + txtID.Text;

debug your code here

Select * from tblStudent where ID = " + Convert.ToInt32(txtID.Text);// if id int

or

Select * from tblStudent where ID = '" + txtID.Text+"'";

best way is
debug code
and check strquery
copy and paste in sql and execute  if your query is wrong then edit it

Try like this
da.Fill(ds); 

or

da.Fill(ds,"tblStudent");