I'm trying to run a new ethernet cable through a conduit, but it feels blocked. What's the best way to get it through?

When a new cable won't pull, the conduit is likely overfilled, has a sharp bend, or an internal obstruction. First, try using a specialized cable pulling lubricant; it significantly reduces friction. If it still won't move, resist forcing it, as you risk damaging both the existing and new cables, or even the conduit itself. The existing 'stuck' wire indicates a deeper problem: either the conduit is past its allowable fill ratio, or there's internal damage like a collapse or debris. You'll need an electrician or general contractor to properly diagnose the blockage, possibly using a camera snake, or to evaluate if a new, larger conduit run is required.