Black Screen

I can’t boot my deskstop (pentium 733 Mhz with window 98 SE), even with a startup disk (floppy):I just have a black screen on the monitor (I sure that the monitor is working proply, because I try it on another computer).I realise that the power supply fan start to work for only 2 or 3 mn, then stop working.
Tis computer was shutdown for more than a year.
What can I do and what is the source of that probleme.