Hi Cantona and
Is this as its booting up before the text appears on the screen ?
If so its usually caused by the memory or graphics card, one or the other may have gone faulty or maybe they have become dislodged slightly and just need reseating.
Or it could be the fan not running for the cpu.
I assume that you havent added any new bits between it being ok and this problem developing ?
Do you know which type of BIOS your pc is using or can you tell us which make and model of pc you have ?