Did you try a different port on the router? They can suddenly go bad. I recommend you verify it really is the cable first. And sadly, so is rodential and hungry 6 and 8 legged critters. Teething puppy dawgs are common culprits. The cable (or one of the connectors) must have been damaged, perhaps through crushing (from walking on it, or setting heavy furniture on it), or being yanked/tripped over one too many times. I will note that network cables don't just stop working. If there is an open anywhere, current stops everywhere. Current flows through a 'circuit' - a word that implies a 'closed' loop. The problem is, current doesn't just run up to a break (open) then stop. That won't solve your issue with the cable running behind bookshelves. There are wire trackers, but you almost have to be on top of the wire across its whole length. None that I know of - unless you are willing to spend a fortune.
