Windows 10 Troubleshooting SrtTrail Error After System Restore: Access Denied to Fixboot

If most of the Advanced Options in the Recovery Environment are missing, this is caused by faulty Boot Configuration Data. If the blue screen lead you to use those commands, it is probably SYSTEM_THREAD_EXCEPTION_NOT_HANDLED caused by CPU core and memory limits in the Boot Configuration Data.

If it's a UEFI system, Bootrec /FixMBR and Bootrec /FixBoot are irrelevant as the UEFI firmware directly reads the partition table and filesystem to run Windows Boot Manager without using bootsectors. Even if it's legacy BIOS, the bootsectors are static unless corrupt or overwritten by something like a Linux installer. It is the BCD.

Bootrec /RebuildBCD only locates Windows installations that are not already in the BCD, and adds them. No installations already in the BCD are changed by Bootrec /RebuildBCD unless the existing BCD file is renamed (which can only be done in the Recovery Environment, which may be missing Command Prompt because the BCD), or unless the existing Windows entry is deleted from the BCD with bcdedit /delete {default} which can be inside Windows, immediately before using Bootrec /RebuildBCD to add it back. The BCD file should be backed up before doing that, in case Bootrec /RebuildBCD fails somehow. Windows should not be restarted between bcdedit /delete {current} and Bootrec /RebuildBCD success.

So, in Command Prompt from inside Windows (like Safe Mode) or Recovery Environment, use
bcdedit /export C:\Temp\BCDold && bcdedit /delete {default}
Bootrec /RebuildBCD || bcdedit /import C:\Temp\BCDold Input A when asked
This way, the existing entry is only deleted if the BCD is successfully backed up.
The backup is automatically restored if a new entry isn't successfully added back.