Try repairing the GRUB bootloader with Boot Repair. Boot to an Ubuntu live session from the live USB/DVD that you used to install Ubuntu. While running an Ubuntu live session open the terminal and type:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt update
sudo apt install -y boot-repair
Open the Boot Repair application and select Advanced Options -> Main Options tab -> check Reinstall GRUB and Unhide boot menu as shown in the below screenshot. Click the Apply button. Then reboot the computer with
If the OSs were installed in different modes, dual booting Windows and Ubuntu can't work. If your Windows is installed in BIOS mode, it is recommended to install your Ubuntu in BIOS mode, but if it's installed in UEFI mode, then do the same with Ubuntu.
BIOS vs. UEFI
To check whether your computer has UEFI or BIOS firmware open the terminal and type
dmesg | grep "EFI v"
This will return a line like this, if the system was booted from EFI:
[ 0.000000] EFI v2.00 by Lenovo
Or it will return nothing if it was booted from BIOS.
If you have installed Ubuntu in legacy mode on the same drive with GPT partitioning, you can use Boot Repair's Advanced options to uninstall grub-pc and install grub-efi-amd64. That converts the Ubuntu installation from BIOS boot to UEFI boot, the same firmware as most recently manufactured laptops with Windows pre-installed have.
Converting Ubuntu into UEFI mode
- Start Boot-Repair, and select Advanced options -> GRUB location tab.
- If you do not see a Separate /boot/efi partition option, this means that your PC does not have any UEFI partition.
If you see a Separate /boot/efi partition option, put a checkmark in the checkbox to the left of it, then click the Apply button in the lower right corner.
Set up your BIOS so that it boots the hard drive in UEFI mode. The way to adjust this setting depends on the specific model of the computer, but generally this setting is located in the boot priority settings under the Boot tab of the BIOS/UEFI setup utility.
For more information about converting Ubuntu into UEFI mode review https://help.ubuntu.com/community/UEFI in the section about Converting Ubuntu into UEFI mode.
The grub bootloader can also be converted in the opposite direction from UEFI to BIOS. Linux can boot fine from a GPT disk in BIOS mode. See this answer: Convert from EFI to BIOS boot