On power up, can you press ESC or DEL or whatever key sequence gets you into the boot manager? I upgraded a LattePanda running lubuntu 16.04 to 18.04, and it was looping through some failure sequence. When I pressed ESC and entered the boot manager, I found that the first item in the boot sequence was an entry for Android. I don't have Android installed on this device, so I don't know how or why that entry was there. I selected the override to boot ubuntu, which worked fine. Once in the ubuntu O/S, I entered the following, based on info I found in another post:
sudo modprobe efivars
which told me that the Android entry was #5, so I then entered:
sudo efibootmgr -b 5 -B
to remove that entry from the boot sequence so that the next time I powered off and back on, it went to ubuntu.
Hope this helps.