I've made a dual boot in a Dell Optiplex 7020 with a existing Windows 10. Now Ubuntu is booting directly, grub doesn't seems to load (or if it loads it doesn't show nothing in the menu) and I can't access BIOS/UEFI. Dell logo doesn't appear anymore and using the keys that I used before doesn't do nothing. Also, I can't boot from a USB stick as I can't do anything with BIOS/UEFI. I have no clue what can be done...
First you need to start grub with command
grub in a terminal. This allows you to see what grub does and how it starts up all the systems.
root (hd -then hit the <Tab> key to auto-complete. once you are logged into grub. This will show you a number of disk options, and once you select the disk you want, with a command looking like this
grub> root (hd0, you should be able to see all the partitions of the harddisk selected.
For a list of all commands look here: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/3/html/Reference_Guide/s1-grub-commands.html and use what is required to modify what you startup.