open xml reading from excel file

Your approach seemed to work ok for me – in that it did “enter the loop”.
Nevertheless you could also try something like the following:

void Main()
{
    string fileName = @"c:\path\to\my\file.xlsx";

    using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
    {
        using (SpreadsheetDocument doc = SpreadsheetDocument.Open(fs, false))
        {
            WorkbookPart workbookPart = doc.WorkbookPart;
            SharedStringTablePart sstpart = workbookPart.GetPartsOfType<SharedStringTablePart>().First();
            SharedStringTable sst = sstpart.SharedStringTable;

            WorksheetPart worksheetPart = workbookPart.WorksheetParts.First();
            Worksheet sheet = worksheetPart.Worksheet;

            var cells = sheet.Descendants<Cell>();
            var rows = sheet.Descendants<Row>();

            Console.WriteLine("Row count = {0}", rows.LongCount());
            Console.WriteLine("Cell count = {0}", cells.LongCount());

            // One way: go through each cell in the sheet
            foreach (Cell cell in cells)
            {
                if ((cell.DataType != null) && (cell.DataType == CellValues.SharedString))
                {
                    int ssid = int.Parse(cell.CellValue.Text);
                    string str = sst.ChildElements[ssid].InnerText;
                    Console.WriteLine("Shared string {0}: {1}", ssid, str);
                }
                else if (cell.CellValue != null)
                {
                    Console.WriteLine("Cell contents: {0}", cell.CellValue.Text);
                }
             }

             // Or... via each row
             foreach (Row row in rows)
             {
                 foreach (Cell c in row.Elements<Cell>())
                 {
                     if ((c.DataType != null) && (c.DataType ==           CellValues.SharedString))
                     {
                         int ssid = int.Parse(c.CellValue.Text);
                         string str = sst.ChildElements[ssid].InnerText;
                         Console.WriteLine("Shared string {0}: {1}", ssid, str);
                     }
                     else if (c.CellValue != null)
                     {
                         Console.WriteLine("Cell contents: {0}", c.CellValue.Text);
                     }
                 }
             }
         }
     }
 }

I used the filestream approach to open the workbook because this allows you to open it with shared access – so that you can have the workbook open in Excel at the same time. The Spreadsheet.Open(… method won’t work if the workbook is open elsewhere.

Perhaps that is why your code didn’t work.

Note, also, the use of the SharedStringTable to get the cell text where appropriate.

EDIT 2018-07-11:

Since this post is still getting votes I should also point out that in many cases it may be a lot easier to use ClosedXML to manipulate/read/edit your workbooks. The documentation examples are pretty user friendly and the coding is, in my limited experience, much more straight forward. Just be aware that it does not (yet) implement all the Excel functions (for example INDEX and MATCH) which may or may not be an issue. [Not that I would want to be trying to deal with INDEX and MATCH in OpenXML anyway.]