If you dual boot a Linux install alongside Windows 10, you may run into an issue where the Windows 10 BitLocker (full disk encryption) panics and refuses to boot your computer. (For example, if you make a change to the BIOS setting, and BitLocker thinks the hard drive has been moved to a different computer.)

The downside to this is that when BitLocker Panics, it may do so before loading the GRUB 2 bootloader, instead popping up a screen asking for your bitlocker “key”. [You DID save that key file in a safe place, right?]

However, even if you do not have access to your BitLocker key, it is usually possible to access (boot into) your Ubuntu partition (If you have a modern UFEI computer) . There are two possible ways to do this. The first is directly from the BitLocker key recovery screen. You can pick to “skip” this drive, and pick an alternate boot option, and one of the options will likely be your Linux/Ubuntu partition (because the UFEI system still knows about the Ubuntu boot system, even if the Windows Bit Locker has over written the Grub master boot record.

Alternatively, on my laptop at least, you could also interrupt the normal boot sequence and pick the “f12 – pick boot device” menu and the Ubuntu/Linux partition was one of the options (along with booting from a USB drive, etc…)

Once you have Ubuntu booted up, a simple:

sudo grub-install

in a terminal should get the GRUB MBR back where it belongs.