No idea, my crystal ball is somewhat dusty at the moment. If you think, this might be a overheating problem, then please have a look at the temperatures of your system, also at the fan speeds.
After the system had shut down, please touch the CPU cooler. If it is rather cool, then the thermal contact is probably not okay. In that case, the cooler must be dismounted, and thermal compund needs to be reapplied (after cleaning CPU and cooler surface, of course).
Also, make sure, the fan is spinning after the computer has been turned on.
If so where can I get a fan and how do I install it, or is there something else wrong? Thanks.
CPU coolers and fans can be bought at computer stores. Very often, installation instructions are enclosed.
But before you go and buy a CPU cooler or a replacement fan, you need to find out what the problem exactly is. Could be also a bad PSU, bad RAM, bad motherboard (look at the capacitors --> http://en.wikipedia.org/wiki/Capacitor_plague)