I am unable to post new messages or submit assignments to a class that has ended.

A course will automatically enter a dormant or archived state approximately two months after the course end date.  This state will not allow any new posts or assignments to be submitted by the students or instructor.
 
Once the course enters this state, is not possible to restore the ability to post.

The student and instructor may have to work out an alternate means of communication and assignment submission. 

There is no additional information.