Boot error after screwing up dual-boot files
Solution 1:
I will recommend you to first reinstall GRUB using a Live USB/CD of Lubuntu and then repair your boot drive by using Boot Repair. Follow the steps on link provided and then use bootrec.exe to achieve your goal.
You can follow these steps
Installing Boot-Repair
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
Using Boot-Repair
Recommended repair
launch Boot-Repair : type "boot-repair" in terminal.
Then click the "Recommended repair" button. When repair is finished, note the URL (paste.ubuntu.com/XXXXX) that appeared on a paper, then reboot and check if you recovered access to your OSs.
If the repair did not succeed, indicate the URL to people who help you by email or forum.