It is super important if they still have access to any PHI or ePHI. You may try to reach them either physically or remotely and make sure they understand the termination.
However, if you still can’t reach them, as long as you have something on your end that has been disseminated to accounts receivable and other parties who may need to be aware of this termination, then you are OK.
It is also advisable to send a certified letter to their last known address, notifying them that their contract is considered terminated. Keep a copy for your records. The key is DOCUMENTATION, DOCUMENTATION, DOCUMENTATION!!!