Yes they can restart an assessment, although not recommended, except in an emergency or power outage.
To restart they simply use the link on their original email invitation.
The system will detect that they are restarting and will ask them to confirm that they wish to restart.
IMPORTANT: If they stop in the middle of a test then the scores for that test will not count and the system will restart at the beginning of the next test / question. Every restart is logged.