Black monitor

Hi
When i switch on my PC i hear a long Beeeep and the monitor stays blank.
I have checked all cable connections and all are in place. My graphics card is OK, the DIMMs of memory (64mb/128mb)are OK, and the monitor works
All the above have been tested.
The other thing is that the PC switches off on it’s own after a couple of minutes.
The spec is: P2 400Cel, TNT2 G.card, 192MB, 6.5GB
Any suggestions please.
Thanks,Yiotie