The Windows boot manager is actually a piece of small software that helps you load your system.
It is part of the volume boot record, which is responsible for the boot process.
In this case, you gotta find solutions to fix the error.

Stay Calm When Windows Boot Manager Failed
When you find theWindows boot managerfailed, yo dont panic.
Error: Inaccessible Boot unit, How To Fix It Yourself?
The most possible cause of this error is a damaged or corrupted Master Boot Record.

Definitely, you are not able to slide in your system when Windows Boot Manager is failed.
In fact, the root reason is a corrupted Master Boot Record (MBR).
Whats more important is how to fix when you find Windows boot manager not working.

It is actually located in the root directory of theactive partition.
In most PCs, it has aSystem Reservedlabel.
When it is damaged, you wont be able to boot into the system.
You must follow the solutions given above to have a go at troubleshoot Windows boot manager failed issue.