First time I use Forums. For -I don't know what- reason, Excel do not ask me if I need to save a document before closing. Any idea how I can modify that?

It used to do it until yesterday, but today, nothing. So, even if I make changes into an existing document, (and let's say I did not save the changes by clicking "save"), Excel will not ask me if I want to save changes before closing.

Please help.