If your windows installation vanished after installing Linux
I had windows 11 installed on sda (hard disk) and Manjaro Linux installed on sdb (SSD). For some reason I reinstalled Manjaro on the same drive (sdb), and windows boot entry vanished.
I can see both EFI partition and C: partition exist and accessible but BIOS firmware settings or grub doesn't detect windows bootloader. efibootmgr shows windows bootloader but it removes windows bootloader when saved.
Solution
Use another windows computer or make a virtual box VM with windows and use Rufus to make a bootable windows USB. Boot up your computer using that, and select [Repair your computer] instead of installing.
Try "Startup repair". But it didn't work for me alone.
Open command line from windows recovery menu and type
BOOTREC /FIXMBR
BOOTREC /FIXBOOT
BOOTREC /ScanOS
BOOTREC /rebuildbcd
Then restart. When restarting boot up from the windows USB again, go to repair and run "Startup repair". Restart again. Hopefully you'll boot into your existing windows installation.