If you are quick enough, you may be able to recover the assignment via your school's "test" or "beta" environment. These two environments are separate from your school's Canvas "production" environment...which is the environment that you are typically used to logging into throughout the week. I'll provide you with a couple links on how to access your school's "test" and "beta" environment. Just be sure to log out (How do I log out of Canvas as an instructor?) of whichever environment you are in before you return to your school's normal "production" environment.