My MacBook Pro won't start

I found my MacBook Pro in a frozen or sleeping state. Nothing on screen. So I held the power button down to force a shutdown. After all activity stopped and I was sure it had powered off, I pressed the power button to power it back on. Nothing. It was unresponsive. No sign of life whatsoever. I tried the power button repeatedly but to no avail. I tried pressing the power button while simultaneously pressing Command, Esc, etc. Nothing. No signs of life. What has happened? What can I do?