Fujitsu Siemens Lap Top

Hello,
I have a Fujitsu Siemens Lap top Pi 1505 which is 3 years old.It boots up fine each day with no problems.
When the lap top is left for more than 24 hours the power button doesnt boot the lap top up.The only method that seems to work which i have found by trial and error was to plug the lap top into the mains with the mains power off and hold the power button down on on the lap top and switch the mains power on and off until eventually it boots up.why does this only happen after 24 hours idle.

Regards

James Turner