Why can't I edit this Numbers spreadsheet?

I have a Numbers spreadsheet which I created on my Mac. My problem is that when I open it, it will not allow me to make any edits. I can see all the rows, columns, etc. including the content of each cell.... but I cannot select any of these cells.

All of the header items such as Function, Table, etc., are present but they are dimmed and I'm unable to select them.

I have other Numbers spreadsheets which allow me to edit just fine. My problem is with this one particular spreadsheet.

Is there some type of setting that I have unknowingly set whch has locked this particulare spreadsheet?

Any help is much appreciated.

Thank you.

Numbers 3.0.1

Mac 10.9.1