All LEDs start blinking!!

I am using a PII-233 Mhz with R440LX dual processor motherboard & Windows 2000 professional OS. But if I don’t operate the PC for 2-3 Hrs it goes in suspend mode & all the Keyboard LEDs start blinking & after hitting the space bar or touching the mouse the monitor screen comes back but I can’t type my password the LEDs continues to blink, I can’t even reset the PC by pressing the reset button the only option is to power off & again switch ON the PC. So what could be the reson for this? Can anybody help me out?