Updating individual packages shouldn’t be an issue, although some people lose video. If you did an upgrade to a new version of Ubuntu itself, this is always fatal.
For those who can still log in, this would tell you if any critical files were damaged:
sha1sum -c /etc/nv_tegra_release
In all cases where you no longer have access and want to see what’s going on you’ll probably have to have a serial console set up. See:
http://www.jetsonhacks.com/2015/12/01/serial-console-nvidia-jetson-tx1/
(serial console is the same on TX1 and TX2, except that TX2 does not support CTS/DTS flow control…TX2 is software or “no” flow control)
If you did do an upgrade to a new Ubuntu version, then you’ll need to flash and start over from scratch. You’re not going to be able to save it. If you have critical data though you can clone the root partition for later reference.