Hello, I had this problem a couple of times, it didn't bother me but i would like to know what caused it. I used Linux Mint 9 Xfce as usual surfing the web, etc. and don't think i pressed any keyboard shortcut. Suddenly mint boot splash screen appeared and I couldn't get out of it, I waited but nothing happened so I had to reboot. Could this be a hardware/ overheating problem? The PC is kinda old (512MB RAM) but works ok apart from being loud.