Here are methods by which you can communicate with students in a circumstance where your classroom is no longer available.
Email is obviously crucial, especially in the early stages of modifying your course (the ASAP Announcement and perhaps Modified Syllabus). But attempting to use Email to operate an online course can quickly get out of hand. Consider D2L your core toolset for course content, assignments, and learning interaction between course members.
Efficiently email your class using D2L or Exchange Faculty Email Accounts.
Every course in D2L has a News Feed. This can be more efficient for you and students than relying on email.
For many disciplines, message boards (Asynchronous Discussions) in D2L can be a quick way to move interaction online. Students can discuss something without specific dates and times that may no longer be practical. (And you may find quiet students in your face-to-face class have more to say in text discussion!)
Dr. Jennifer Stowe has assembled a great set of variations on student discussion activities. These will get students creating, and thinking about how they learn.
Asynchronous discussions are often a default method in online education. But you can achieve social learning and teaching presence in your courses with a variety of other tools. Any of these methods may better serve your learning objectives, provide students with creative opportunities, and foster a sense of community within your courses.
D2L has a simple (elegant, even) chat system that allows realtime text communication among class members. It's easy to install and quick for your students to learn. It's perfect when you want to discuss something at a real-time pace but video web conference is more than what's needed.
Short videos, even just screencasts where students hear your voice, can reassure them them that you are engaged in their learning, despite circumstances.
Record Screencast Videos for your students.
Record Webcam Videos to Communicate with Students - Transcript Short webcam videos can be great for quick announcements, short concepts, and in general reminding your students that you are engaged in course, despite circumstances.
This is an option, but consider carefully whether live online classes are the best option.
Google Hangouts Meet. Simple and easy to use.
Zoom. Allows recording of meetings. Make sure to protect yourself from "Zoombombing"!