It may be as a result of bad power supply or your fans have a problem and needs to be replaced with new ones. You need to switch off your computer by unplugging the power cable then plug it in to see if the fans will stay for a longer time. You can also contact a dealer or a computer technician to help you identify the problem and fix it.
Could simply be your fan is about to burn out. If it is a PC tower, unplug it and take a vacuum to it. Put a pair of woman's hose over it to make sure you don't suck in any parts though and don't have the vacuum right on top of anything. If that does not help...you may need a new fan.