It probably means that when you try to create order occur unexpected error. You could turn on display of mistake and reproduce the error, if it will create you will see it. It can do in index.php at Magento root:
Change next code:
if (isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
}
#ini_set('display_errors', 1);
For this:
//if (isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
//}
ini_set('display_errors', 1);
If it is life site, will be better turn on error log in admin System > Configuration > Developer > Log Settings > Enabled = Yes after that cheek in yet one time (reproduce the error) and look at logs. Old logs (var/log/system.log, var/log/exception.log, and folders var/reports) could rename (or move).
If you will find error these ways, complete its your question and we try to help you.
display_errors = 1
at .user.ini check the request output. you got to know the error reason.then you can be able tosolve this