This weekend I dusted out my computer with some compressed air. When I was done I booted it up and it... just... wouldn't. The boot sequence was different, it offered some different options and they didn't work -- regular Ubuntu option didn't work and when I tried to run a memtest it would just stop at 35%. Eventually I selected an option which chugged for a while before producing
I had no idea what this meant and the internet suggested that either the disk was corrupted or the memory was bad. It's a solid state drive and I hadn't touched the RAM chips. I was ready to start pulling components but instead I went back and blew it all out again -- I got underneath, I sprayed up and down the memory chips and the SATA connectors.
When I tried again it booted like normal. So add "dust" to the list of things to check out.