I've recently built a new computer, and it's sitting on my desk right now as of posting. I've put everything together correctly, and my system boots (with no appearance of BIOS, which may be because of the new HDD.) with no abnormal sounds. However, the computer won't respond to holding down the power button to turn off.... Any ideas? thanks for any answers.
Well, its clear you are moving too fast. Your OP talks about the power button when the real problem is it doesn't boot. Then you post "Eureka! I found it!" when you find the cpu power cable not plugged in, and later discover it still doesn't work.
Slow down, and go over the list again.
Then make sure you match "post beeps" you hear to their meanings. You may need to buy/install a motherboard speaker to do that. Without it, you'll have a lot more guessing to do.