If you want to keep pecking away with the wrong tools, chop a patch cable in half, strip the wires, and plug it in (with the other end of the cable unplugged) and verify no connection from any wire to any other wire.
Then strip the wires on the other half, and twist each pair together, plug it into the jack at the far end and verify continuity of each pair from the other end.
That's the sort of thing you can automate/do more easily with a $10 ethernet tester; or laboriously with a cut patch cable and a multimeter.
After that your twisted-pair ethernet wiring tester options move to "hundreds or thousands of dollars" and replacing the cable is cheaper.
If both ends of the cable are connected to jacks (as is proper) start by pulling them out and re-punching the connections into the jack (for which you need a punchdown tool, barring weird tool-less jacks.) If the other end is a crimped-on plug, that's instantly your suspect (and you have a harder time checking continuity) as plugs crimped on wall cables fail with some regularity. Replace it with a jack and a patch cable. You can try replacing the jacks, but unless you see an obvious issue by visual inspection, jack failures are relatively rare.
Edit: You state in a comment that "One end has to be crimped as it connects inside a low-voltage box similar to OnQ" - by using a surface-mount jack enclosure you can, and should, use a jack (and short patch cable) there as well. They come 1, 2, 4, 6 or more - I show a 2 below. The surface-mount box can be installed inside the "central smart box" - a good "central smart box" would have punchdown (like the jacks) patch panels available for this purpose, but a generic surface-mount will work. "OnQ" does seem to have 6 or 8-port punchdown patch panels available. I show an 8 below. They are rather expensive and space inefficient, as they leave the punchdown out and that takes up space. Little 12-port patch panels with the punchdowns on the back (you have to dismount them, install the cable, and remount them - which you do only once, or at most once per new set of cables) are available for generally considerably less cash, and take up less space in your box. The last item shown below.
So, at that point, you are likely better off replacing the cable, rather than spending a large amount of money for a tester you will use rarely or never that will most likely tell you you need to replace the cable.
One hack you can try (after having tried simple re-termination) which might get you from no connection to 100Mbit (better than nothing, and likely plenty for most TV uses) is to connect "the wrong colors" to your jacks. If you have no connection, the 1,2 & 3,6 wires (orange and green pairs in TIA568B scheme) have a problem. If you connect the blue and brown pairs (4,5 & 7,8)as if they were orange and green (so to pins 1,2 & 3,6) you might get a functional 100Mbit connection if the cable damage is confined to orange and green. The signals don't actually care what color the wires are, so long as they are paired with each other correctly.
If not "poor connections at the ends failing" the usual suspect is "rodent chewing cable damage" in a concealed location. While there can be other causes of cable damage, you state no construction activity, and that one is all too common.