probs with desk top

[teletext]

could anyone help me please, my desk top started playing up the other day,switching itself off just after boot up. The screen looses signal, now when i turn it on the green power light by the turn on switch is on then eventualy the red light under the green illuminates, then after about 2 minutes the fans start i.e. cpu & graphics fans. but unfortunately thats it both red and green lights are constantly on but apart from that the computer seems dead! sorry this is a bit long winded but i am surely stumped.
many thanks in advance
teletext