|Open the side of the computer and then turn it on while watching it, You should have a CPU fan/heat sink and a rear mounted exhaust fan (desktop) and both should be spinning up at start up. If one does not start then you will need to replace it. If it is the case fan at the rear then nearly anyone can easily replace it, just measure the diameter in mm (sizes like 80mm, 120mm) and purchase one from a local computer store. There are typically 4 screws holding it attached (possibly plastic clips) and one 3 pin connector to the motherboard. Just make sure of the fan's direction (blowing out) and make sure you ground yourself (touch a metal switch plate screw or metal faucet) to prevent static electricity from damaging internal components. If your CPU fan is bad then it will depend on how technical you are since most of the time you will need to replace the entire CPU heat sink which means carefully removing the heat sink, cleaning off the thermal compound that is on the CPU and reapplying the proper amount of compound with the new heat sink (exception, some original equipment heat sinks come with preapplied compound).|
See here for instructions:
or... take it in for service if you are not comfortable doing this.
You have to be a little bit crazy to keep you from going insane.