If you are trying to boot your Cloudbox VPS and you receive the following notification in the control panel: Virtual machine cannot be started at the moment. (Not enough RAM on HV for boot virtual machine.)
You can migrate your VPS to another node. Go to your VPS in your Cloudbox control pane, click 'Tools' at the top right and then click 'Migrate Virtual Machine.
Subsequently you can choose a different node with enough free space. After selecting the node, click 'Start Migration'. The VPS will immediately be migrated, after which you can boot the VPS the way you're used to.