Called a Kernel Panic; often caused by a hardware failure, particularly defective RAM (but not always). However, it can also be caused by a one-time kind of distruption and never reappear. If the problem doesn't reappear, don't become concerned over it. If it does reappear, have the extra RAM you had installed removed and re-test. Kernel Panics can also be caused by corruption of the OS or other software.