Find and fix cause to Something failed in Power Automate
If you've set multiple flows for document reminders in SharePoint, you'll need to take a close look at each flow to find the one that failsYou will receive the following email from Microsoft Power Automate (powerautomatenoreply@microsoft.com). The email contains no further information except the text Sending reminder: Something failed.

Possible Cause – Deleted SharePoint Library
The cause may be the deletion of a SharePoint team site or the deletion of a SharePoint document library for which you had set a reminder with (Power) Automate. In this case, the explanation is that when a document library is deleted, a flow for a document due date or expiration date reminder is not automatically deleted. Power Automate continues to run the flow for this, but no longer finds the document library for it.
If you've set multiple reminders using Automate in SharePoint, you won't be able to identify the right flow from the Sending reminder: Something failed email.
Error analysis in Power Automate
Flows for reminders are in the list of Cloud flows, under My flows at https://make.powerautomate.com. In the execution history, the status is always Successful, even for flows for reminders in deleted document libraries.

You also can't see the failed flow in this list. To identify the broken reminder flow, you need to take a close look at each flow individually.
You can only see that a reminder flow fails because the document library has been deleted in the details of the respective execution: The step Scope to get items due in x day is marked with a red warning.

Under step Get list name by HTTP request it says Notfound. Check the URL in the Site Address field.

Use a web browser to check the URL in the field under Site address – you'll probably see a 404 Not found HTTP error. Compare this address with the address of the deleted SharePoint sites or document libraries.
Learn more
Unfortunately, Microsoft Power Automate doesn't send a hint, such as the name of the reminder flow. If you have multiple flows, it is therefore time-consuming to identify the faulty one. Vote for the idea More information in email notifications when something fails · Community – Ideas need at least 50 votes (see Ideas need at least 50 votes before they are reviewed - Microsoft Power Platform Blog).