Turnitin have issued the following announcement about an issue users may have encountered on the 18th, 19th and 20th July, 2015: 

“Some users may have encountered an issue with the Turnitin and TurnitinUK services on the 18th, 19th and 20th of July, 2015.
Images and buttons within the Turnitin service, including the assignment submission page within Turnitin may not have loaded correctly.
This issue may have prevented student users from being able to submit work.
The issue was resolved on the 20th of July, 2015 at 01.15 PST. We apologize for any inconvenience that may have been caused.”

Further information about Turnitin Service status is at:

http://www.turnitinuk.com/en_gb/support/system-status