It would help to know what system you're asking about - if is a desktop, one of the fans - either the chassis fan or the one in the power supply - is faulty.  You'll nee to find out which and replace the fan or the power supply.

I used to get the thermal event message after trying to restart an Inspiron 1100 laptop that had shut itself off.

Replacing the heatsink / fan unit solved the probelm. When I did so, I used Arctic Silver thermal grease, which some say is better than the stuff Dell uses.....