txtItemDescription = (TextBox)GridView1.Rows[rowIndex].Cells[1].FindControl("ItemDescription");

private void SetPreviousData()
{
TextBox tbobject = new TextBox();

int rowIndex = 0;

if (ViewState["CurrentTable"] != null)
{

DataTable dt = (DataTable)ViewState["CurrentTable"];

if (dt.Rows.Count > 0)
{

for (int i = 0; i < dt.Rows.Count; i++)
{

//tbobject = (TextBox)GridView1.Rows[rowIndex].Cells[0].FindControl("RowNumber");
//tbobject.Text = dt.Rows[i]["RowNumber"].ToString();

DropDownList itmDesc = (DropDownList)GridView1.Rows[rowIndex].Cells[1].FindControl("ItemDescription");
itmDesc.Text = dt.Rows[i]["ItemDescription"].ToString();

rowIndex++;

}

}

}

thank you. its is done. I was not declaring dropdownlist in page load..