Can not save workbook

I constantly work on large workbooks with 30+ sheets. The workbook was bult by copying a format sheet and modifying the newly added sheet. Many times when I tried to save the work an error msg showed up - "document not saved". If I tried to move the sheet to a new workbook, I got a msg of "permission denied". I can only highlight the sheet and do a copy and paste to another workbook. The help msg indicated the workbook was protected or write-only. But it wasn't the case. What's the problem? How do I fix it?
Thanks