grub efi – Dual boot, Ubuntu not booting after Win11 update

I’ve looked for a recent answer I can understand (absolute novice) but nothing seemed to address my specific problem.

My system is set to boot Ubuntu first (see setup screen image). The second boot is Windows 11 home, fully updated as of 2024/09/21.

Each time I switch on I get the following:

Failed to open \EFI\UBUNTU\grubx64.efi - Not Found

Failed to load image __: Not Found

start_image() returned Not Found, falling back to default loader

Failed to open \EFI\UBUNTU\grubx64.efi - Not Found

Failed to load image __: Not Found

start_image() returned Not Found

It then boots into Windows.

Please explain like I’m a kid, how I can make my laptop boot into Ubuntu again?

I should add that I bought it a while back as-is, and I have no install disk for either Windows or Ubuntu.