Did you ever have to start from zero again, career wise?

Depends on what you mean. I have not started over as in changed fields & career totally, although that is something I am contemplating for the future.

I have started over as in I climbed the mountain of tech roles to CIO, then I was a founder for a few different companies, a couple that I successfully exited and then I started over as a senior engineer at a company and now am the CTO. I actually love being just an engineer but also love being CTO or a product person, mainly because I really enjoy the team building aspect and solving hard problems.

What I can say is be careful on your expectations & motivations, starting over and being a novice in a new field will feel different than starting over and having been already successful in a field/industry. Check the career growth, salary growth of the positions/field before you jump because something I discounted was the lack of monetary growth can significantly demotivate you. At least to me, and I think most people in tech, money isn't the only motivating factor, but the lack of progress monetarily sure can demotivate you quick.

Sort of, for some value of zero. After a couple of decades of steady work in tech, I had a personal, overwhelming circumstance arise that I knew would kill my productivity (or worse). I resigned my job and did what I needed to do to get through it.

Then started hunting for a new job. As I came to realize, having quit a job without first lining up a new one does kind of put you at zero.

Did eventually find a nice (albeit low-paying) job. No regrets, though. I did a great thing for myself. And I suppose I do feel a certain righteous schadenfreude every time I hear hirers complaining that they can't find anyone good anymore.

I was an expert at Visual BASIC 6.0, ADO, ASP 3.0, SQL Server 2000, Access 2000, then Dotnet came out and people started to switch to Python as well.

I have to learn all over again. I know the basics of a language as they use the same logic as others. So I can write small programs in Python, but I have to reinvent myself.

I am 51 years old, and it seems I am too old to hire. Especially when I have a disability. So I get overqualified for most job postings and no response for others.

Yes. Best quality of life choice I ever made. I was in my mid-forties, too. My experience has been overwhelmingly positive.

I studied theoretical physics then switched over to software development.

I would like out of tech. no idea what to do to start over.

I was a graphic designer, then I did cabinet making, auto sales, manual labor and construction. I was also in the military. I finally got into tech and I don't always love it, but I remember those years when I froze my ass off working on a jobsite somewhere making peanuts, and I'm thankful. TLDR I've started from nothing several times. If I had to do it again right now, I think I would open a coffee shop.