How do I boot into a resized partition?

Grub won't find the start of the Ubuntu Mate partition anymore, because it has been moved. How do I proceed? I tried to boot into an Ubuntu Mate USB Stick and chroot into the partiton to do "update-grub", but it says "Cannot find list of partitions! (Try mounting /sys)". I think that's because it's booted from USB. How would you fix a Grub after the only Linux partition you had has been moved (so that the beginning sector is now a different one)?

when you chroot make sure you mounted all fs.
Example: (this assumes UEFI, /boot/efi in sda1, / in sda2)

sudo mkdir -p /mnt/boot/efi
sudo mount /dev/sda1 /mnt/boot/efi
sudo mount /dev/sda2 /mnt
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt

grub-install /dev/sda
grub-install --recheck /dev/sda

Thank you, it worked perfectly! I can boot again.