How to dump the boot log of the jetson nano device?

We have previously successfully tested several Jetson Nano into the Ubuntu system, but after a few days. Unfortunately, we found that there is no HDMI display after several Jetson Nanos are powered on. We have tried: (1) Rewrite the system to the TF card (2) Replace the power supply to 5V4A (or USB) (3) Replace the power adapter and TF card. Now we want to get the boot log of the device through UART or other methods, we appreciate any helpful comments and solutions.