suppose my CAS server is down OS crash so I need to install OS and select SCCM recover  site? it will work? or else need to install CAS and select SCCM source file for site recover which method will work

Good Morning SCCM Ghost

Yes, you will need to install the OS, SQL and recover your site. As with any recovery it is always recommend that you contact CSSS directly for support, particularly when you have a CAS. Since this mean you have over 150,000 clients.