Probably Limit Exceeded or 0 recipients Exception in Salesforce

Possible scenarios are
1. If you send an email to the user who is inactive.

2. If the Email Alert is set to Owner and Owner is Queue with no members in it.

Generate Debug Log and find the email alert recipient value.

Best Practices:
1. Make sure to check whether the user is active in the Process Builder, Flow, Workflow, etc before sending the Email Alert.

2. Don't have queue where there are not active Users.