This worked for my team
- Database credentials embedded in workbooks expire. (Alerts require workbooks to use either embedded credentials, or none at all.)
- A data source becomes inaccessible.
Have you checked for the credentials ?
If nothing is working for you (from above link) then it will be good to have a case created for Tableau support team. They can assist you.
It turns out that the SMTP settings were not configured properly in the Tableau Server Configuration. Working now. Thanks.