Won't wake up.

A few times my iMac has been in sleep mode and when I come back it won't come out of sleep mode. I have to hold down the power button until it reboots.
Is this common or do I have something strange going on.
I am using leopard and have the update installed.
Thanks

That's strange. Are your sleep settings default? That or the power source may be messed up.

The settings are changed but it also did it before I changed them. Doesn't seem to matter the amount of time, the first time it did it it had been in sleep overnight and the other time it was in for only a short time.