I installed Ubuntu 16.04 in an Acer R5-571T 59DC notebook for dual booting with the preinstalled Win 10 (UEFI). After restart there is no grub menu and the pc boots directly on windows. I disabled secure boot and fast boot before install, and also used "boot repair" with a live usb. No luck. I managed to start Ubuntu using Super Grub2 Disk, so I think grub is ok but without the super grub pendrive I cannot boot ubuntu. Your help is appreciated.
Upgrade, update the system, and install grub. After that your problem will get solved.
sudo apt-get update
sudo apt-get upgrade
For installing grub, follow this link How do I manually install GRUB?
I found a good enough solution. I enabled secure boot again in the UEFI settings and after that the option for adding the new OS to the EFI boot menu was enabled. Now when I access it with F12 key I have both the Ubuntu & Windows options. Thanks wjandrea for your answer