If Dtab.Rows[0]["Privilege"].ToString() this returns Text then try to use
ddlprivelege.Items.FindByText(Dtab.Rows[0]["Privilege"].ToString()).Selected = true;
If Dtab.Rows[0]["Privilege"].ToString() this returns ID then try to use
ddlprivelege.Items.FindByValue(Dtab.Rows[0]["Privilege"].ToString()).Selected = true;
And you have to sure Dtab.Rows[0]["Privilege"].ToString() value should exists in dropdown from your text or id of inserted data