It's not adding much, and not helping much given your tragedy has already occurred, but right next to the well (or as close as you can, and where there's still full access to the site), you can try to build (then fully cancel) the following, in turn:
- A soap-maker's workshop[1], to checking that you have both a bucket and one of the accepted 'block' items available.
- A restraint, to check that you have a rope/chain available.
- A lever, to check that you have a mechanism available.
I know it's just the block and chain it mentioned (which also rules out another strangeness I can think of which should have prompted you about everything, if not given another error altogether), but the above should work well.
Alternatively, designate four 1x1 stockpiles right next to the well site, each one
dedicated to an individual component (you can zero the bin/barrel numbers), although you'll never be quite sure whether the dwarfs are just not hauling there, rather than that they cannot or they have nothing to haul.
Sorry, I'm also going down the "are you
sure you've got them all available!?!" route, but it really is the most likely thing. I've done worse, in the past, although once I knew for sure what I'd done I was then able to recognise the symptoms again.
[1] Asheries and Dyer's workshops would also work for the bucket, but also need a barrel (superfluous to this test), and I think only one of them needs a block or pre-block equivalent material, so you'd also need to (b)uild a (c)onstruction to get a handle on the block availability.