Sounds like the nut in your jack screw is shot. They eventually wear out anyway. Often, they are fairly soft bronze. If you are in the habit of running it up or down until it hits the hard stop, it dramatically accelerates the damage. If that's the case, you're hosed and will likely need at least a rebuild. Often, those are more expensive than a new one, and take much longer. You may be best off buying a new one and installing it. In the meantime, a bottle jack, or the jack from your TV may work to keep the tongue off the ground while you replace.
2006 Jayco Seneca 34SS
2006 Jeep Wrangler Unlimited Rubicon Toad