Black screen on dell inspiron

Hello, I have a Dell inspiron 1521 purchased about two years ago. Windows Vista Home premium. I was on the internet this morning and stepped out for a smoke only to come back to a black screen. The power light was on but all LED lock lites were out. I have done the Dell diagnostics on their website and by pushing and holding the FN key while powering up I get the two left LED Lock lights flashing and the power lock lite is constant. Can this only mean a new mother board or is it possible there is something else causing this condition? I have tried swapping memory sticks but still same response two left flashing one right constant on the LED's. Also noticed the hard drive sounds like it starts up but within two seconds stops. Fan is running and vents are clean but still seems to run very hot. Thanks for your help!

Fan is running and vents are clean but still seems to run very hot.

What do you mean by "still" and what do you mean by "very"?

Do you have an old CRT or a flat panel with VGA connector.
Hook up as an external Monitor, to help trouble-shoot whether this is maybe related to your Display.

Thanks for responding Doug! By "still" I mean the light flashing sequence did not change after trying the memory sticks one at a time. By "Very" hot I mean hotter than other laptops I've felt. More in the center area on the bottom side. This laptop has always seemed to run hot though so I have never thought it to be a problem. I did try hooking it to an external monitor but nothing came up on it. The laptop is less than three years old and I have not changed or replaced anything since purchased. The power light still comes on on the right side and the fan still runs. If I put my ear near the hard drive it sounds like it starts when first powered on but stops within two seconds. Also after doing the FN power on test, I heard the fan surge like it got a brief boost of energy for a second or two then went back to its regular speed that it normally runs. Could a faulty fan cause this black screen? Thanks again for any help.