What happens if you use @reboot reboot in crontab file?
Testing result:
If you add
@reboot reboot
This did not work!!! Why?
$ which reboot
/sbin/reboot
Now When i added the full path of reboot, the cron job runs normally.
So you got an infinite loop of rebooting in the case:
sudo crontab -e
And add the line:
@reboot /sbin/reboot
EDIT:
If you want to get rid off this, you probably need to boot from an Live CD, then mount your disk drive and go to the cron file and remove the added line.
sudo gedit /media/some-UUID/var/spool/cron/crontabs/root
Save and then remove livecd and reboot again, and you'll be safe