Have a look in /var/log/maillog see if you can spot any errors.
What port is your imap server running on? Is it running?
"netstat -A inet -nlp" or similar might show you if your imap server is running.
In roundcube main.inc.php config file, what do you have $rcmail_config['default_host'] and $rcmail_config['default_port'] set to?
Does roundcube config match your imap config?
I have dovecot listening on ports 143 and 993. However, my firewall rules dicate that outside users can only access IMAPS on 993. Roundcube running on the same machine is set to connect on 143.