If your servers are down or have to go down during business hours you should notify the users at least 15 minutes beforehand so you will not get 101 people all asking you if the computer is down.
For short outages that only affect a few people, IM is the best method. If you use Teams or Skype, simply open a new chat window, click the "Invite" button, Select All the recipients, and then hit OK to add them to the Message window. This is best used for short outages and when you have a small number of users, or if you will be performing maintenance out of hours and you expect users will not be using the affected systems. If they are not online on Live Messenger or Skype, they can't complain that they were not warned.
For extended or planned outages, or if you have a larger number of users (50+) Email is the suggested method.
If you send an email it is a good idea to tell the user a way to monitor the network themselves. Eg. Software solutions like SCOM or WhatsUp Gold.
Include a "To Myself". It gives visibility to others who are interested in what needs to be done to fix the problem and makes
it easier to remember to send the done email. E.g. "done - CRM is alive again".