I use Ubuntu (a modern Linux distribution) as my main work station. Everything worked fine until I upgraded from version 14.10 to 15.04 of the OS. At this point strange things started to happen. After working for an hour or two my environment would freeze and stop responding altogether. Neither Ctrl-Alt-Del nor Ctrl-Alt-Backspace would produce any reaction. I couldn't connect to the machine remotely as well. Only hard reset would take me out of this state.

I couldn't find anything useful in system log after reboot. No kernel panic messages, no noticeable errors. And there were no specific symptoms other than the actual freezing. Neither was I running some peculiar or even heavy applications. I mentally also ruled out a possibility of a virus - those try to abuse your machine, not to make it freeze.

I also ruled out hardware faults. Firstly, the machine was stable before upgrade to Ubuntu 15.04. Secondly, I have a Windows partition on the same box and it worked just fine.

After some searching and reading Ubuntu/Linux forums, I started to suspect that there could be a problem with my video drivers. I have an NVidia graphic card. After checking the drivers in usage, I realized my machine had enabled open source ones. I switched to the proprietary drivers provided by NVidia itself and, voila! everything started working fine and there were no more freezes. End of story. And they lived happily ever after :)