Can a person write a thesis years later if illness set in? Can a person write her thesis years later for the mere sake of doing it if illness interrupted her upon completion of graduate courses in English Literature?
It is probably possible, but would depend on the university and an advisor. Unfortunately, some places have time limits and old courses don't count anymore. But in the case of illness, it might be possible to get exceptions to such rules. 

But it is a question for a particular place and a particular advisor. 

But that is assuming you want a degree. You can, of course, write anything you like and possibly get it published even without a degree.