locked spreadsheet

hello,
I have excel 2007 and win7. It seems I have locked the spreadsheet and can't edit an old spreadsheet. I can't enter new values into existing or new cells. Nor can I enter values into the white box near the function key at the top of the page. The file name at the top of the page is illuminated in a new manner to me. I have tried to open other old sheets with same results. There is also a "compatibality issue" posted beside the illuminated file name at the top of the sheet. I have never seen this. Any ideas on how to unlock the sheet?
thanks