How do I close a Libre file which is locked open?

I mistakenly clicked on the lock icon in the header list instead of the print icon.

Sorry. I do not know a lock icon, and the help doesn’t mention it. I also do not know the header list. Again the help doesn’t help. Would you mind to explain?
A ‘Print’ icon is located in the standard toolbar by default. Did you click on another icon next to it in the same toolbar? Was the toolbar customised?