My kneejerk guess would be the amplifier, irrespective of the fact that you say the TV does not appear to have been affected. You have to take into consideration that many modern TVs have fairly sensitive front-ends on them, possibly good enough to suffer a dramatic drop in gain from the amp and still give a good picture.. Barring an amplifier problem, I would look for moisture inside the coax fitting(s) at the antenna (including Balun, if equipped) or at the ground block. Also look at the antenna for damaged elements. If by chance you've gotten a shot of lightning, you could also have developed a carbon path between center conductor and ground at the ground block, or even across the insulator(s) on the antenna "driven" element. For multiple output connections to have concurrently lost a lot of signal it has to be something common to all receivers. Also if it's a Rat Shack splitter, I have seen these go bad or become intermittent.
As part of annual "routine" I open up all coax fittings at least once a year to check for possible corrosion & clean the threads accordingly , as needed.. Then apply a very light coating of silicone grease to the threads and then tighten things back down. If you have a small torque wrench, the coax connectors should all be tightened down to 30 inch pounds. I do this for OTA and cable/sat as well.
Also, coaxial cable that is exposed to the elements will eventually go bad & develop attenuation distortion. I do not have a recommendation for how often to replace it, but I try to replace mine every 7~10 years.
Also when installing coax you want no kinks, no staples (ie, staple-gun), no sharp corners and no tie-wraps that have been cinched-down. Snug is OK but not tight.