Can't boot Ubuntu after setting up partitions manually during installation
You need to install the boot loader on the hard disk device (/sda) and not on the boot partition. So select your hard disk device for "Device for boot loader installation".
My first suspicion would be that the boot loader isn't where it should be. Note that it needs to match what device your BIOS is set to boot. If that is still the disk or stick you used to install Ubuntu....
You can also boot in to a live instance, perhaps the same one used to install, to take a look around and figure out what happened and maybe re-install your boot loader.