It is a terrifying moment: you turn on your PC expecting the familiar purple Ubuntu boot menu, but instead, you are greeted with a black screen, a cryptic 'error: unknown filesystem', and the dreaded `grub rescue>` prompt.
Don't panic! Your data and your Ubuntu operating system are almost certainly safe. This happens when the GRUB bootloader loses track of where your Linux partition is located—usually caused by resizing partitions, a Windows update overwriting the bootloader, or a corrupted boot file.
Use this interactive diagnostic tree to determine exactly which GRUB error you are facing, and execute the precise commands to repair your bootloader and get your system running again.