If it's been overheating for that long a period of time, it's likely you have done some damage to the mainboard or the hard drive.
Was the fan working?
If not, start with a replacement fan and a thorough cleanout of the dust on the heatsink. Replace the thermal pad that sits between the CPU and heatsink as well. If the system won't power up thereafter, it's likely you've damaged the mainboard and will need to replace that as well.