My wife's HP laptop dual boots Windows 10 Pro and Ubuntu Mate 20.04.
We recently updated Windows, and it appeared to also update the BIOS. Following the update, we no longer get the Grub menu on boot, and so we can't boot into Linux.
How can I fix this without completely reinstalling Ubuntu Mate?
Hi Robin, welcome to the community
Your Windows 10 update demolished GRUB
(no worries, this has been the standard MS-Windows behaviour for decades, we are used to it )
What you need is the 'Linux boot repair '. This is what I use when GRUB has been demolished by MS-Windows:
https://help.ubuntu.com/community/Boot-Repair
Quote:
Boot-Repair is a simple tool to repair frequent boot issues you may encounter in Ubuntu like when you can't boot Ubuntu after installing Windows or another Linux distribution, or when you can't boot Windows after installing Ubuntu, or when GRUB is not displayed anymore, some upgrade breaks GRUB, etc.
Boot-Repair lets you fix these issues with a simple click, which (generally reinstalls GRUB and) restores access to the operating systems you had installed before the issue.
Thank you. I know I've had this problem before, although not for some time. I thought I'd fixed it previously by booting into a Linux installer and selecting an option to repair the existing installation. If I did, that option has obviously been removed 
Boot-Repair fixed it for me, though.