Please read: See the last bit. I don't believe this is an issue related to Ubuntu, but rather VS Code.
Recently, under 17.10 and 18.04 (I just upgraded today), Ubuntu will randomly and completely freeze. I cannot move the cursor or use the keyboard. Naturally, I have tried switching to a TTY session with no success. Every time, I must resort to SysRq + REISUB, which is (obviously) not preferred.
I have attempted to switch graphics drivers, on the theory that it may be that. I was originally using the open source nVidia driver, and switched to the proprietary to no avail. I'm currently using the open source driver.
All that I typically have open is Firefox (not a ton of tabs loaded) and VS Code, with at most a couple files and a terminal open.
Update: The system logs show no indication of anything at all happening, let alone something wrong. I don't know if this is related, but sometimes (and especially after booting) programs take a significant amount of time (> 5-10 seconds), even command line programs.
Yet another update! — Even with all GNOME extensions disabled, it still crashed. I tried XFCE, which still crashed.
Summary: Ubuntu completely freezes at seemingly random times, leaving no trace in logs, and is not related to any specific desktop environment or GNOME extensions.
(likely final) update: I'm convinced this is an issue with VS Code. I have been running GNOME for days, with other Electron apps open (Slack, Pulse, etc.), and have not had a single freeze. I have DM'd VS Code on Twitter, and will likely file a bug report as they haven't responded yet.
journalctl -b -1 -xewill show you logs at the end of the previous boot. How much RAM do you have? How much swap?
free;swaponwill tell you.