I don’t recall intentionally locking any of he documents in question.
If documents aren’t actually locked by a running process (not necessarily LO, it may be e.g. another office suite like Calligra of MS Office), then it might be a leftover lock file that is a hidden file named .~lock.<original-file-name.ext># placed in the same directory as the original file. It might be left if LO crashed while the file was open, so no normal cleanup happened.
Usually, LO allows you to open this file anyway (that’s one of options in the confirmation dialog), ant this option should take care of this stalled lock file. If, however, for some reason this fails, you may remove this file safely yourself, if you enable showing hidden files in your windows manager.
Found the hidden file and deleted it. This is the second time i have had this problem and the second time that this solution has worked. Yeah LO!
Menu/File/Reload.