While your computer is booting, hold shift
. If you see message GRUB Loading, you know you did it well.
In the GRUB menu, see if you have more than one OS. If you do, boot the one you want to, and if you don't, well it is gonna take more time.
Case 1
Your chosen system will boot, and you will be happy :)
Case 2
If you don't see the second OS, go to the OS you see (probably Lubuntu) and when it boots, run in terminal
sudo update-grub
It should find the second OS and add it to fstab file.
When it's done, reboot and try to get to GRUB again. If you still don't see second OS, then it looks like the second one is not bootable. You might have to get GParted Live and copy all your data somewhere safe and reinstall.