I accidentally deleted permissions to a list for all users (including myself - the admin)
I want to get access to the list back
I broke the permissions on the library level
There is no way to recover the custom permissions you granted in the past or the permissions the library had before remove them. What you can do is to inherit again permissions from the parent site. You can go to "permissions" settings for that library and click on "Delete unique permissions" to start inheriting again. Please, read carefully this article from Microsoft Support to understand permission management in the right way.