Hi Jan, it can't be truly locked, as the software doesn't support locking. Any message about a page being "locked" is a just a soft warning that somebody else has recently opened the page in edit mode. You should be able to override that warning and save the page.
What happens is that spiders crawling the web will sometimes open the page in edit mode (fortunately they don't save), so sometimes you'll see that locked message. The "locking" times out after a few minutes.