Spreadsheet document opens in "read only" mode, need to be able to add data.

I have an open office spreadsheet that I need to add data to. The problem is that it is opening in "read only" mode and will not let me add any data to the spreadsheet. This is a file that I burned onto a cd for storage, which I do continually as I update the file with new data. However, I had lost the file on my computer (of this spreadsheet) and had to transfer the data from cd back onto the computer. After I did that I started getting this message, that the "Document opened in read only mode", whenever I attempt to add data. Normally, I open the spreadsheet and add data immidiately by entering it in. I have never had this problem before.
Thanks for your help.