Plants are over 40 years old, have never been separated, and now are "disappearing" - dying off. Should I separate them now or when they appear in the spring, or will they come up again at all?
Hosta are said to never need division. However, the soil beneath them can get depleted of nutrients. 2 inches of compost under the leaves annually should keep them going. If the plants have been stressed this year by abnormal weather or too much sun, they may go dormant in early fall to protect themselves. But curiosity is a good thing. I would dig one up and examine the roots; is there a mole/vole eating roots, is the soil staying soggy too long and roots are rotting. See if there is a fixable problem. I am pretty confident that your hosta will emerge next spring.