To repair a non-booting Server 2012 R2, you cannot rely on the OS itself. You will need:
Once in the Command Prompt, run these commands in order to fix standard boot sector issues: Microsoft Community Hub bootrec /fixmbr windows server 2012 r2 boot repair
30–60 minutes
If it fails, manually assign a letter to the system partition. To repair a non-booting Server 2012 R2, you
Startup Repair often fixes BCD errors, missing boot files, and some registry corruption. If it fails, proceed to manual methods below. missing boot files