If you were using the IMAP4 protocol for reading, your mails are probably unharmed, and using your service provider's webmail interface (check your documentation where you can reach it) should verify this. On the other hand, if you were using the older POP3 protocol, unfortunately the messages are typically NOT left on the server after having been download from it unless you've instructed the reader to do so. I always recommend people to use the webmail interface whenever they can, just for these reasons.