Just because you're not using them as an ISP doesn't mean you can't access the email account. At least when you used them as an ISP you should have been able to get your mail via POP3 or IMAP from outside their little network.

It's possible that whilst you cancelled your service, the mailbox just kept running along.

Do they have a webmail server you can use (like gmail) to at least log in and delete some messages and perhaps setup a forwarding email address if you want.

Why can't you contact them from your gmail account? My ISP does use my contact from address to automatically assign support requests to my account but that's about all. Unless they actively block all emails originating from non-ISP email addresses you should still be able to contact them.

What's the ISP?