When resizing your VPS, it will be stopped, modified and started again. No data will be lost, except when shrinking the hard disk, which will require a reinstallation of the server.
In the event insufficient resources are available on the physical host where your VPS is running, you will be notified that the VPS has to be migrated to another host. This migration will take some time, in which your server will be unavailable. The amount of time depends on the disk size, the configurator will show the estimated time. After migrating, the resize will be performed and your server will be started again.