Outlook not working can be caused by a mix of local device issues, account problems, or service outages. Here are practical steps to diagnose and fix common causes. First, determine if the problem is local or service-wide

• Check if Outlook (or Office 365) services are down for everyone. Use a status page or outage trackers for Outlook/Microsoft 365. If there’s a known outage, you’ll just need to wait for it to be resolved [outage guidance].
• If only your account is affected, you might see sign-in problems, password issues, or security blocks rather than a complete service outage [outage guidance].

Common quick fixes you can try

• Restart and update
  • Close Outlook and all related processes in Task Manager, then reopen. Ensure Windows and Office are updated to the latest versions; updates often fix bugs that cause startup or performance issues [outage guidance].
• Run Outlook in safe mode to disable add-ins
  • Start Outlook with the safe mode switch to see if add-ins are causing the problem. If Outlook works in safe mode, disable problematic add-ins one by one and restart normally [outage guidance].
• Repair Office installation
  • Use the built-in repair tool (quick repair first, then online repair if needed) to fix corrupted files that might block Outlook from starting or functioning properly [outage guidance].
• Check mail profile and PST/OST health
  • A corrupted profile or mailbox data file can cause startup or sending/receiving issues. Create a new profile to test if the problem is profile-related; run a scan of PST/OST files if you suspect corruption [outage guidance].
• Connection and capacity checks
  • Verify your internet connection is stable. Ensure your mailbox isn’t overly large or near quota, which can slow or block syncing. If using cached Exchange mode, try turning it off to test direct server connection [outage guidance].
• Antivirus and firewall interactions
  • Temporarily disable antivirus email scanning to see if it’s interfering with Outlook. If this resolves the issue, adjust your antivirus settings rather than leaving protection off [outage guidance].

If you’re unable to resolve, gather what to provide for help

• Exact symptoms: is Outlook not opening, stuck at a screen, not sending/receiving, or slow?
• Any error codes or messages (e.g., 0x800ccc0f, “Cannot start Microsoft Outlook”), and when they appear.
• Your Outlook version (Microsoft 365 App, Outlook 2019/2016, web Outlook) and whether this occurs on one device or multiple.
• Whether other Microsoft 365 apps are affected, and whether you’re online or using a VPN.
• Whether there’s a recent change: new add-ins, updated software, or password changes.

If the issue appears to be on the service side

• Check for announced outages or service health alerts from Microsoft or your IT department.
• Consider using the web version of Outlook temporarily while the desktop client issue is being resolved.

Would you like me to tailor troubleshooting steps to your setup (Windows/macOS, Outlook desktop vs. web, single device vs. multiple devices), and include step-by-step instructions for any of the fixes above?