Hello Reihaneh,
Earlier today Lsync, a software used to synchronize the PrepCast website across multiple servers, crashed without triggering an alert or error message. Because of this crash, user login-information wasn't correctly shared between the servers making it in effect impossible for users to login and use the website/simulator.
But because there was no alert or error message about the crash, it took the system admins longer than usual to identify the problem, which went away once Lsync was restarted.
Everything is back to normal now and you may continue your access. We apologize for any inconvenience this may have caused you.
You may now log in and take your final exam.