Cause: This symptom is typical of overheating. Overheating processors are a major source of PC freezes and crashes. Other than the main processor, the graphics processor can also overheat and bring the entire PC to a grinding halt. Like the CPU, the graphics processor has its own heat sink and fan. An overheating graphics processor would point to some problem with the cooling system.
Solution: Troubleshooting this is similar to troubleshooting an overheating CPU. However, you should eliminate all the other heat generating culprits first: check the power supply, the CPU heat sink, and fan. Once you have eliminated these possibilities, turn your attention to the graphics processor: check the heat sink and fan, it may be that the fan is not working or the heat sink is not fitted snugly on the graphics processor. Check the power connections for the fan and reseat the heat sink to fit it tightly with the processor. Use thermal paste if necessary.
Such graphics cards should have a temperature sensor that can be viewed from within the operating system. This would give you a good indication of whether the card is overheating. Unless you are stressing out the system with high-end 3D applications for hours on end, it is unlikely that the graphics card is your source of trouble. In most cases it would be the power supply. But the sudden freeze and shut down is typical of overheating, so look at the CPU and PSU again if you have ruled out the graphics card as the source of the problem.