To reboot the server, go from your personal area (https://my.llhost-inc.eu/clientarea.php) to the Server Management Manager:
Personal Area > Services > Click on the service with Dedicated Server > Login to Control Panel.
You will be redirected to the DCImanager interface, where you can find your server (https://dci6.llhost-inc.com/dci/server/list).
To reboot the server, click on the drop-down context menu and click on "Reboot":
DCImanager will ask you to enter the Server ID to confirm the reboot: