What do you do with socks who's elastic is gone?

Well I think that is a sign that you need to retire them and find a replacement for it. It is so annoying that your socks slides back and you have to pull it out every now and then. Remedies I see when I was a child was placing a rubber band at the end so it will hold on to your legs. But that is aesthetically not good to look at. I think you can always recycle those old socks in to something like a rag so that you can utilise it pretty well.

I turn them into hats for my cat.cats with beanies are cute.

I throw them away because it means it's time to buy new ones.

You can put garters on the upper side of both socks. I also do the same thing if see that the appearance is still looks good.

I usually throw them. It is not comfortable to wear them loose

I don't know if it can be fixed, I'll advice you buy new onces because you might be embarrassed one day if you are a office person.