A cold boot is a restart of a computer system that occurs when the system is powered off. When a computer is turned on, the BIOS (basic input/output system) initializes the hardware and starts the operating system. When a computer is turned off, the BIOS saves the state of the hardware to RAM (random access memory) and then powers down the system. When the computer is turned on again, the BIOS restores the state of the hardware from RAM and starts the operating system.