If you upgraded to 5.1.4, check this out.
http://bugs.cubecart.com/view.php?id=328
Is yours OK? If not, there is a temporary fix suggested:
Temporary fix
classes/hookloader.php
line 166
if ($enabled) {
$plugin = $GLOBALS['config']->get($basename);
if (!$plugin['status']) continue;
}
comment out