Hi Chloe
Sorry to hear about your fall. This would potentially fall into the civil law area of Negligence, whereby McDonalds could be held to have been negligent in not following proper procedures to ensure you were protected from sustaining injuries while on their premises. There have been cases run in the past where a retailer was held to be negligent after a customer slipped in store on area where there was a spill, even though the staff had put up a wet floor sign.
Did the staff there get you to fill out an incident form? Either way, I would be notifying McDonald's as soon as possible about your fall and advising them you will be seeking legal advice/ looking into your legal options etc. I would then consider getting some legal advice from an injury law firm and having them look after this for you. If you don't want to go down that path, you can always try to negotiate a settlement with McDonald's directly. Given they are such a large corporation, they likely have a standard process for responding to these sort of claims.
Just ensure you keep a diary of your injuries each day, id go to a doctor and get your injuries assessed and keep a copy of all of your receipts.
I hope that helps!
Anna