I completely understand how inconvenient it is to have the "My Templates" add-in disappear suddenly, especially when you rely on it to speed up your workflow.
Based on my research, it appears this is part of a known issue currently affecting Microsoft services (specifically identified as Service Incident EX1238375).
Please refer to the following article: My Templates are missing from Outlook
To put it simply, this is a system-wide issue on Microsoft's end (server-side), rather than a glitch on your specific computer. Because the problem is affecting the connection to the add-in service for the whole organization, it unfortunately cannot be manually restored by individual users right now.
I recommend contacting your organization’s IT Administrator. They can verify this incident in their system and open a support ticket with Microsoft to track the fix. You can refer them to this guide: Get support - Microsoft 365 admin | Microsoft Learn
While waiting for the fix, you can use the Signatures feature in Outlook as a backup. You can save your frequent text templates as different "signatures" and insert them quickly into your emails. It functions very similarly to My Templates.
Reference: How to add and change an email signature in Outlook - Microsoft Support
I hope this information is helpful, or at least clarifies why the feature is missing. Please feel free to reach out if you have any other questions or need further clarification.
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