bootrec /fixboot : Writes a new boot sector to the system partition.
:If rebuildbcd fails to find an installation, the BCD store might be too corrupted to read. In this case, you must manually move the old store and create a new one: windows server 2012 r2 boot repair
If the server starts booting but fails (e.g., a Blue Screen), try booting into Safe Mode bootrec /fixboot : Writes a new boot sector