Classic sign of either a major dust build up or (more likely) the processor heatsink compound/paste has dried up and has caused the heatsink to break away from the processor.
You need to take it apart or take it to a professional repairer.
don't keep turning it on as the temperature can significantly reduce the processor life span.
It shouldn't be too difficult to repair if the paste has dried up and the laptop will function perfectly from then on (in almost all cases).