Been using for years. New Dell laptop and I left the MS OS on the computer and loaded Ubuntu 16.04.2. I then switched at boot to Microsoft, and now the computer will not reboot back to Ubuntu. How do I get back to my Ubuntu OS!?!?
windows likes to put itself at the top of the boot order. a work arond is this:
boot from a live CD. install efibootmgr.
find the file /efi/ubuntu/grubx64.efi. copy it to /efi/boot/bootx64.efi. if the file is there already overwrite it. this makes grub the default loader.
next use efibootmgr to delete all entries in the boot list. restart you computer, be sure ubuntu is the fiirst OS you boot into so it is placed in the first slot.
Windows likes to take over the booting whenever possible, and doesn't recognize Linux partitions.
You'll need to boot from a live CD or live USB and reinstall GRUB. This will make GRUB the default boot loader again, and should recognize both Ubuntu and MS Partitions.