This will depend on who the sequence was set to send from. You may want to go into those sequences that are active, and created by users that you deactivated, and change the 'Send Email From' settings.
Also, if you have the sequences set to 'send follow-up emails as a reply in the same thread', those contacts will no longer get emails since the system will not be able to look up that thread of the connected email address of the user that was deactivated. This can be resolved by turning off the toggle to 'send follow-up emails as a reply in the same thread' under the sequence settings.