You need to go far back in time to remember a CPU microprocessor that was able to operate completely without a heat sink. Intel's first Pentium CPUs were already producing a considerable amount of heat, but the specifications allowed operation without any special heat removal. A little bit later processors required at least a passive heat sink for trouble free operation. However, for the last three years is has become state of the art that a CPU requires a heat sink as well as a fan that ensures reasonable air flow through the cooling fins.
The reason is quite simple. The high clock frequencies of today's processors, which are nowadays measured in Giga Hertz and not in Mega Hertz anymore, lead to a heat dissipation of 50-80 W. To remove this huge amount of thermal energy from a CPU you require large and highly sophisticated solutions. Those heat removal solutions need to be attached to the microprocessor extremely closely. Once the heat conducting connection to the CPU is lost, the heat cannot be dissipated anymore and the processor overheats, which leads to different scenarios.
Common Reasons For Heat Sink Failure
Not everybody assembles his PC by his own. Some people wouldn't even contemplate opening their PC case to take a look inside. Those PC users are typically buying complete systems, either in a big retail shop, or from an online mail order company. In both cases the PC needs to be transported to your doorstep, either by yourself or by a delivery service. It is no rare occasion that the processor heat sink fell off while the system was in transport. The result is a black screen when the system is started for the fist time. In some cases this problem can easily be solved with a simple re-fixation of the heat sink, but often enough the processor did not survive this first doomed run of the system. A replacement of the processor is required, which can cause a lot of hassle, should the retailer not be willing to do this replacement free of charge.
Computer enthusiasts are of course not afraid to fiddle around with their computer system. Many of them are also trying to squeeze some more performance out of their processors via overclocking. Those folks are fully aware of the fact that the cooling of the CPU is extremely important. Therefore they often invest in expensive huge heat sink solutions. Those large heat sinks have one big problem however. They are often extremely heavy as well. If the mounting mechanism for those devices is not very sturdy, the heavy heat sink can still fall off because the mounting mechanism breaks off.
The most common problem with today's heat sinks however is fan failure. A lot of the cheaper heat sink/fan combinations use very cheap bearings for the fan spindle. Together with the dust that collects in the fan the worn out bearing suddenly stops the rotation of the fan. The airflow around the heat sink drops down to almost zero, so that it doesn't take long until the processor has reached a temperature that won't allow reliable operation anymore. In the worst case the CPU takes damage.