Fans are blocked i thinks
You'll need your trusty can of compressed air. Start by powering down your PC, removing the case lid, and locating the various fans. Starting with the power supply, blow through the internal slits from inside the chassis, aiming so dust will exit the back.
Next, blow into the intake fan (if there is one) to push more dust out the back. Finally, blow the blades of the rear exhaust fan clean. If possible, aim just beneath the center, where the motor meets the fan assembly, and blast again. Repeat the process for each fan, keeping the can upright at all times.
Now restart your PC, and while the fans are spinning, spray them once more--very briefly--to really send the dust flying.