Problems with dell inspiron not booting

I have a dell inspiron that all of a sudden won't boot up.I had previously shut down in the normal way but now when I turn it on the fans start-stop-start-stop-start again and I can here one beep. After that -nothing.There is nothing on the monitor at all and no amount of pressing F12 etc helps. i have removed and re-inserted the memory modules and even tried another hard drive .
Any help greatly appreciated but I am guessing that maybe I have a motherboard problem.
thanks in advance