I had this problem a few days ago, which took me a while to sort out since nobody in my university's usual suspects could provide me with a solution (in fact all solutions took me to the same guideline on how to set up a class). I finally found a helping hand in my university's e-learning who told me that the account was inactivated due to a period of inactivity. Since I use Turnitin to set up individual classesfor my PhD students to check their thesis, is pretty obvious that I do not access Turnitin every day and there will be periods of inactivity. 

I am not sure if the zeal to cut access to professor/instructors from Turnitin after a "period of inactivity" is something that Turnitin does or it is unique to my university. None of the two possibilities, especially the latter, would surprise me. 

To come back to your problem, the most expedite way to sort it out would be to contact e-learning in your university (or an equivalent service) and ask them to check if you account can be activated. Then, clear the cache in your browser, exit the browser, re-start turnitin, and may the force be with you.