A few days ago I made the plunge from Windows and put Linux Mint on my laptop (Thinkpad 440), the newest release. Things are mostly fine, except twice today I've been browsing the internet on Chromium browser, when suddenly and with no warning at all, the system will switch to the login screen background with the clock, but there's nothing I can use to log back in. It's completely unresponsive, no mouse cursor, nothing appears when I type, nothing happens when I shut the laptop lid and open it again. The only thing I can do is hold the power button until it shuts down and I can restart (thankfully Chromium saves my tabs).
Obviously this is not ideal. I can't do any important work if everything might get nuked without warning. What's going on and how do I fix it?