If you can read the files you should be able to pull out the system journal onto another machine and look at the last logs prior to it dying. It’s basically the same process as detailed out in this guide:
The only difference is you aren’t starting from the ssh addon, you’re starting from the sdcard mounted in another machine. Journal files are all still in the same place though, var/log/journal relative to wherever you mounted it.
I would also guess sdcard given the abruptness and the inability to even start it anymore. There’s not much that can cause a complete failure to even start the machine besides disk corruption. How long were you running it successfully prior to this? Also do you remember if you changed commit_interval or left it at the default?