Excel 2013 no column headers

Hi,

My and a work college needed to combine our separate excel worksheets into a single document.

Office 2013 didn't have a function to "import sheet from file" so we used open office to import my .xlsx worksheets.

After we finished importing we exported the final workbook as .xls (so I could open it).

After opening the workbook on my pc (excel 2013) i notice some of the sheets no longer have column headers, but the row headings still exists.
(No ABCD, only 1234)

Also I am unable to use features such as "Freeze Pane"

I suspect this was caused by importing and exporting through open office?

I tried solving this by googling but it was unsuccessful.

I thought I would post this issue with the forum because it doesn't seem to have a solution.
Thanks