This is what the Windows Alert from Microsoft at Microsoft Community says :
The update to Windows RT 8.1 is once again available as a free download in the Windows Store for users with Windows RT devices. We immediately suspended availability of the update after reports that a very small number of Surface RT customers—around 1 out of every 1,000 Surface RT 8.1 installs—had experienced an issue that left the update incomplete and their devices unbootable. This was due to a rare situation where firmware updates had not completed at the time of the update to RT 8.1. In most cases, if a customer encountered this issue the result was simply an extra reboot. However, for a very small percentage, the boot configuration data was affected which prevented a successful boot. We worked to quickly resolve the issue and now encourage customers to update their Surface RT devices. Surface Pro and 8.1 customers were not impacted by this issue.
As previously announced, there is a recovery solution available for download here for Surface RT customers who experienced the installation issue. We apologize for any inconvenience this may have caused. If an issue occurs with our software or devices, we take immediate steps to ensure a quality experience for every single customer—that’s been our driving priority in this case, despite the very limited number of customers impacted. We are grateful to the customers who shared their Surface RT devices, assisting our efforts to determine the root cause quickly.