Turnitin have reported the issue is now resolved:
Turnitin’s operations team has resolved a database issue that caused service interruptions yesterday. We restored access to the Turnitin UK service this morning at approximately 2 AM BST. We are currently processing a backlog of Originality Report generations and estimate processing times to return to normal around 9 AM BST.
We apologise for the inconvenience this has caused. If you have further questions or concerns, please contact Turnitin UK Support.
Please see the Twitter feed here for Turnitin service status updates.