sfc /scannow /offbootdir=D:\ /offwindir=D:\Windows (Replace D: with your OS drive letter.)
~30% – but when it works, you're done in 5 minutes. windows server 2012 r2 boot repair
For deeper corruption:
bcdedit /export C:\BCD_Backup attrib C:\boot\bcd -s -h -r ren C:\boot\bcd bcd.old bootrec /rebuildbcd (Replace C: with your system reserved partition – often a small 350MB partition without a drive letter. Use diskpart → list volume → select volume X → assign letter=S: → exit then S: for the commands above.) If your server uses BIOS (not UEFI) and you see "Operating system not found": and driver issues
Startup Repair scans for missing/corrupt boot files, registry damage, and driver issues, then attempts auto-fix. then attempts auto-fix.