Won't boot, noisy fan, no display

Yesterday I discovered that the mac mini was still powered up but there was no display. I pushed the power button and kept it pressed to force a power down. When I restart the mac mini, the fan goes on and makes quite a bit of noise, the light at the front remains permanently on, there are never any beeps and the screen never displays. I try resetting the SMC and eventually managed to get it back up and running. This morning the same thing but this time no matter what I try I cannot get it to boot correctly. I tried removing all the memory and then adding it back in into the different slots, 1 at a time then both but still it will not power up. Has anyone else experienced a similar problem and if yes how they managed to solve it. I even tried various displays thinking that this may have been the source of my problem but no display worked.