I can't open any email attachments even though Norton says "no virus found" A note says "Security settings won't allow"

Something has changed with security. Previous attachments would always open. As of yesterday I can't open a very important attachment. I ran a complete security check and Norton says everything has been fixed. Still can't open anything. A note says Security is set to prevent opening of attachments. It's even disallowing attachments I sent to myself. HELP!!