Did you do a masthead switch (moving the clients from one Bigfix environment to another)? I’ve never had my OSD servers disappear in the course of normal upgrades.
Ensure the Analyses from the “OS Deployment and Bare Metal Imaging” site are active, and that the Management Extender and Bare Metal Plug-ins are installed on the Bare Metal Servers.
If it does occur that you need to reinstall the Bare Metal Servers, note you should be able to do this without losing data. The Bare Metal Uninstall and Bare Metal Server Installation do not purge the database unless you run the task to “Cleanup bare metal server uninstall”.