Computers like to stay cool. That's why there generally two fans inside - one for the power supply and the other for the CPU (that's the 'brains' of your computer).
Intel® Processors have built-in thermal protection. If the processor gets too hot, the built-in protection shuts down the processor. If your computer is not overclocked, and is running under the design specifications, the built-in protection can help prevent damage to your system.
Overclocking temperatures could in theory go as high as 90°C while still being 'safe', and the max temperature for many CPUs is listed in the 105-110°C range. But for long-term use, you're much better off keeping things below 80°C in general and only pushing up to 85°C at the most.
This CPU heatsink is only 3 year old, but it is no longer able to cool the CPU. If you pay attention to your computer, you might notice the tell-tale signs:
Computer overheating warning signs
Unusual fan sounds. Most computers have two fans, one fan that cools the power supply, and one that cools the processor. ...
Crashing, hanging, or blue screen. ...
Constantly restarts or shuts down. ...
Increasing or sustaining a high temperature on idle state.
This you can fix yourself, with your own domestic vacuum cleaner, but gently, though.