1. This could be due to overheating of the CPU caused by poor contact between the heat sink and the CPU Remove the heat sink and fan (make sure the fan is working). On the exposed CPU die surface apply some thermal grease or thermal tape. This is to fill in the microscopic gaps that may exist between the CPU and the heat sink. Don’t use too much grease and don’t try to substitute with something else, use approved thermal media only. Also, don’t get impatient and try to force the heat sink on the die to try and make it fit. You may break the heat sink or worse still damage the CPU. If the fan is not working, verify the power connections. If the connections are correct and the fan still does not work you can try replacing the fan but the best option is to get a new heat sink unit (heat sink + fan).
Note: The problem with a failed heat sink is the likelihood that you have already cooked your CPU. Most modern CPUs overheat very quickly and require cooling right from power on.