Some hints to fix booting your favourite OS. Tested with Fedora 25 and Windows 10.

• Boot from live USB
• Switch to root and mount the needed filesystems:

  sudo -i
  
  mkdir -p /mnt/sysimage
  mount /dev/fedora/root /mnt/sysimage
  
  mount --bind /dev /mnt/sysimage/dev
  mount --bind /run /mnt/sysimage/run
  mount -t devpts pts /mnt/sysimage/dev/pts
  mount -t proc proc /mnt/sysimage/proc
  mount -t sysfs sys /mnt/sysimage/sys 
  
  modprobe efivarfs
  mount -t efivarfs efivarfs /mnt/sysimage/sys/firmware/efi/efivars

• Copy the resolv.conf to have some networking in the chroot:

  mkdir -p /mnt/sysimage/var/run/NetworkManager
  cp /var/run/NetworkManager/resolv.conf /mnt/sysimage/var/run/NetworkManager

• Enter the chroot and mount /boot and /boot/efi filesystems:

  chroot /mnt/sysimage
  
  mount /boot
  mount /boot/efi

• Reinstall grub2:

  dnf -y reinstall grub2 grub2-efi grub2-efi-modules shim
  grub2-mkconfig -o /boot/efi/EFI/fedora/grub.cfg

• Check if entry exists in EFI menu:

  efibootmgr

• Add entry if it is missing:

  grep efibootmgr /var/log/anaconda/program.log 
  22:47:35,458 INFO program: Running... efibootmgr
  22:47:35,477 INFO program: Running... efibootmgr -c -w -L Fedora -d /dev/nvme0n1 -p 2 -l \EFI\fedora\shim.efi
  
  #Run the command found above:
  efibootmgr -c -w -L Fedora -d /dev/nvme0n1 -p 2 -l '\EFI\fedora\shim.efi'

• (optional) Add another OS for example:

  efibootmgr -c -w -L Windows -d /dev/nvme0n1 -p 2 -l '\EFI\Microsoft\Boot\bootmgfw.efi'

• (optional) Change the boot order:

  efibootmgr -o 1,0

• (optional) Delete entry:

  efibootmgr -b 0 -B

• (optional) Change timeout:

  efibootmgr -t 2

• Unmount filesystems and reboot:

  umount /boot/efi
  umount /boot
  exit
  umount /mnt/sysimage/sys/firmware/efi/efivars
  umount /mnt/sysimage/sys
  umount /mnt/sysimage/run
  umount /mnt/sysimage/proc
  umount /mnt/sysimage/dev/pts
  umount /mnt/sysimage/dev
  umount /mnt/sysimage
  
  reboot

• Modify the path for the Windows boot manager to Fedora:

  bcdedit /set {bootmgr} path \EFI\fedora\shim.efi

• Alternatively you can use EasyUEFI to edit the entries.