If you are accessing an account via SSH keys it is possible to login to the account but not notice that the password on the account is locked (e.g. due to expiring or invalid password attempts)
If the system is using PAM and the account is locked, this can stop its cronjob from running. (I've tested this on Solaris, but not on Ubuntu)
You may find messages like this in /var/adm/messages:
Oct 24 07:51:00 mybox cron[29024]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local hostOct 24 07:52:00 mybox cron[29063]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local hostOct 24 07:53:00 mybox cron[29098]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local hostOct 24 07:54:00 mybox cron[29527]: [ID 731128 auth.notice] pam_unix_account: cron attempting to validate locked account myuser from local host
All you should need to do is run:
# passwd -u <USERNAME>
as root to unlock the account, and the crontab should work again.