Most food is water soluable, so the hot water should do it. But it takes time. The time can be reduced if you use disk detergent. For more aggressive detergent use dishwasher soap, but that stuff is miserable so don't get it on your skin. Just mix it in the pan and let it soak.
But sometimes the burning process converts it from food to something much harder. If you can get it over there they make a really aggressive material called oven cleaner over here. It is a kind of lye that you spray on. I have used it successfully to clean auto parts. But don't let it get on anything you care about. You might have to heat the pan to get it working.
Finally, you might try a metal putty knife, a blade of less than an inch. Or a single edge razer blade. It will scrape along the flat pan bottom without scratching the pan. Round corners are more of a problem. But the corners may be cleanable by some other means since the pan is less hot away from the burner surface.