Ubuntu 16.04 is installed on UPboard.
It is showing only bios settings after reboot. It is not booting to ubuntu.
It will boot to ubuntu only when pendrive is connected.
Kindly help me to resolve this issue.
You might have installed the bootloader on the USB.
Try to reinstall the system and let Ubuntu manage the partitioning (erase all and reinstall Ubuntu)
I downloaded ubuntu-16.04.6-desktop-amd64.iso image from https://wiki.up-community.org/Ubuntu and flashed the same to pendrive using etcher.
OS successfully got installed but after removing pendrive and rebooting, it is not booting to ubuntu.
Can you try to install Ubuntu 18.04 and see if it is any different?
When you are installing please let Ubuntu automatically Erase and reinstall automatically so that it will install the bootloader correctly.