If you need to stop users from logging in for a period of time, you can switch maintenance mode on. When a user attempts to login, instead of being directed to their dashboard, they will see the below message. You may also include a custom message for your users if you wish to.
Step 1. Login as an administrator and select Administration
Step 2. Select Site Administration
Step 3. Select Server Tab
Step 4. Locate the Server category
Step 5. Select Maintenance Mode
Step 6. Use the dropdown to enable Maintenance Mode (when you no longer require the site to be in maintenance mode, return to this setting and choose the disable from the dropdown box. Users will be able to login again.
Step 8. Add a custom message for your users, this is optional.
Step 9. Select Save Changes
Your site is now in maintenance mode. Users will receive the below message, along with any custom message that you add when they attempt to login. Site administrators will be able to login as normal.