I had to press and hold the Power and Home buttons because my iPhone shut off and wouldn't turn on through normal measures. When I pressed the Power and Home buttons for 20 seconds the iPhone powered on, but does this mean the phone is dying?
If it's a one-off occurrence, it doesn't mean much and you can probably ignore it.
If it's a regular problem, it may indicate something more serious, either software or hardware. You can try restoring the device in iTunes to see if it is a software problem, or take it to an Apple Store.