You can reset an assignment for all students in the class but also just for an individual, if required.
but if all a student has done is to check it as 'Completed' in error, you can also change that as well so he can go back and carry on and will not have to start again from the beginning. Mark all as completed  to see how you can not only check assignments for students as Completed but also where you can 'uncheck' as well