I have a brand new HP Pavilion Desktop which came with Windows, which I detest. I immediately installed Ubuntu Bionic alongside Windows (in /dev/sda6), followed by reinstalling all my favourite apps I had on my old system (MySQL, Thunderbird, Dokuwiki...). After only 3 weeks of using the new system I found it had rebooted itself 4 days ago, despite the fact I have battery backup. I was presented with updates, which I installed. This required a reboot but Ubuntu failed to reboot.
I employed my favourite tool to repair Ubuntu, Boot-repair but, unlike the dozens of other times I used it, it went straight to the Boot Info without allowing me to select the partitions to install grub and all the other steps.
I even tried to reinstall Ubuntu from the installation CD.
I would welcome suggestions on how to proceed. This is the first PC I've had with UEFI and I'm not familiar with it.