Was your sandbox refreshed ? By default when sandbox is refreshed the email access in the settings is set to No Email .

You can set this up in Email settings.

Was your sandbox refreshed ? By default when sandbox is refreshed the email access in the settings is set to No Email .

You can set this up in Email Deliverability to "All Emails"

Set up | Email Deliverability and check whether the sandbox has access to send emails .