If you're signed in but having trouble connecting, check your system's status page for connectivity alerts, maintenances, or other factors indicating that servers are not fully operational. If there are issues with your system's servers, you might experience connectivity issues until they return to full operation.
Clear the Cache of Your Console. Another common reason for the Call of Duty Modern Warfare Multiplayer not working error is the corrupted cache data. In this case, you can try clearing the cache files on your PC or console to fix the issue.