Before trying manual commands, attempt the automated tool provided by Microsoft Support :
bootrec /rebuildbcd – To scan for installations and rebuild the list. 3. The EFI Twist windows server 2012 r2 boot repair
He assigned it a letter, Z: , and formatted the boot files manually using the bcdboot command, pointing the system back to the C:\Windows directory where the heartbeat of the server resided. 4. The Resurrection Before trying manual commands, attempt the automated tool