Difficulty waking up computer

I used to be able to awaken the computer after putting it to sleep by simply
pressing "enter" or "start.

For some reason, now, this no longer works.
In order to awaken it, I now press and hold down the start button on my
computer for 5 seconds and then press it again, which hurts my back bending
over the monitor.
How can I adjust vista so that it starts properly.
I have windows vista business with an Asus motherboard.

Many thankss