To maintain server security we automatically apply operating system updates each night. Most of these updates can be applied on the fly but if there is an update for the system's Kernel module then a reboot will be required to utilise the update. It is not essential to run the latest kernel module unless there is a know vulnerability but you should reboot occasionally to update this component. If you have not reboot in a while, we'll let you know that a reboot is due with an email (if you have that notification enabled).
You can reboot your server via the Nimbus Hosting platform using this article as a guide - https://support.nimbushosting.co.uk/support/solutions/articles/36000020534-rebooting-your-server