Newsletters suddenly stopped working

Mailpoet newsletters had been working fine until two days ago. When I tried to send the new one it did not been sent. A message says that the preview has been sent to my email but there is no newsletter. I tried several times.. nada!

Then, I sent the newsletter to the subscribers and the message said that the newsletter has been sent but actually it did not.

What suddenly changed ? It worked fine until two days ago.

Please tell me what to do otherwise I have to change plugin if this is not working anymore.
Thank you.