If the password has been changed through Directadmin, you can check the logs in /var/log/directadmin/$date.log for entries like this:
/CMD_AJAX_CHECK_PASSWORD HTTP/1.1
POST /CMD_USER_PASSWD HTTP/1.1
However, there could be another reason for not being able to log into Directadmin, for example if your root partition is full (check with df -h shell command) or if you are out of inodes (check with df -i shell command).