There are a few things that could cause boot manager issues. If you are having trouble booting into Windows, you can try a few things to fix the issue.
First, try using the Windows Recovery Environment to repair the boot manager. To do this, insert your Windows installation disc and restart your computer. Then, boot from the disc and select the option to repair your computer. This will open the Windows Recovery Environment, where you can try to repair the boot manager.
If that doesn't work, you can try using the Command Prompt to fix the boot manager. To do this, boot into the Recovery Environment and select the option to open the Command Prompt. Then, type the following commands:
bootrec /fixmbr
bootrec /fixboot
bootrec /scanos
bootrec /rebuildbcd
These commands should fix the boot manager. If they don't, you can try using a boot repair tool like Boot-US.