Most, perhaps all, modern CPUs have a thermistor, and if you go into your BIOS you can configure shutdowns based on temperature. You probably just crossed the factory set threshold and triggered the shutdown that is in place to protect your hardware. When I overclock Android devices, I always throttle back CPU clock speeds at set temperatures to prevent damage, i.e. at 120 degrees throttle back the CPU to 75% OEM clock speed. Oh, and as the other poster mentioned, laptops aren't really that great on your lap. :-)
Answer
Most, perhaps all, modern CPUs have a thermistor, and if you go into your BIOS you can configure shutdowns based on temperature. You probably just crossed the factory set threshold and triggered the shutdown that is in place to protect your hardware. When I overclock Android devices, I always throttle back CPU clock speeds at set temperatures to prevent damage, i.e. at 120 degrees throttle back the CPU to 75% OEM clock speed. Oh, and as the other poster mentioned, laptops aren't really that great on your lap. :-)
Here is a screenshot that shows what I'm talking about.