Work At or ON smth

Could you please explain me the difference between work AT smth and work ON smth?

And what the difference between these two sentences?

You have to work AT a marriage
You have to work ON a marriage

Thank you.

Some suggestions:

to work at - at home, at the computer, at night

to work on (something, a project and so on) - try to fix something

In your examples, I would say:

You have to work on your marriage (i.e. try to make it work.)