Can you hear the fan spinning? If you do, then the problem you have could be due to the problem with the video chip. Sometimes, after much of a use and less cooling the North Bridge - which is responsible for the video signal gets desoldered and its thermic "paste" goes away therefore the screen doesn't show anything but the fans are still running. If you understand notebooks hardware you can do it yourself, opening the laptop and see the chipset that is next to the processor if has been unsoldered or bring it to a technician to get it soldered again.
You have to measure the temperature of your CPU and also get into the BIOS and see the RPMs of the CPU fan and if there's need of that to think of making a replacement of CPU heatsink soon.