How do I clean a "ruined" pan?

Spray oven cleaner on it.

Fill pan with very hot water and dissolve lots of table salt and let it soak over night or for a few days. Scrape off any mess that will come off with a butter knife. Then use steel pan scourer and cream cleaner e.g Cif. Wear a pair of thick rubber gloves to protect hands. Put in dishwasher. Repeat process until pan is clean.

Dump a cup of baking soda into it. Fill it with boiling water & let it cool to lukewarm. Drain the pan and scour it with more baking soda and a damp rag to get the last bits out. Rinse well.

Rachael open a can of tomatoes and put these in

The acid in the tomatoes will loosen any debris

Failing this use baking soda and vinegar bring to a gentle boil

Either of these methods will work for you

I would also use a wire wool soap pad on it

You fix it the same way you ruined it. You cook it clean. Fill with hot soapy water and either boil it on the stove top Or bake it in the oven.

I'd scrape the mess into the garbage can and wash the pan. When it got dry I'd decide if it needed anything further

try oven cleaner, or throw it out, and buy a new one.