Welcome to Apple Discussions!
1. Is it plugged in and is the adapter ring showing any light by the connector's port on the machine?
2. Is the battery showing that it has enough juice that the lights are at least solid?
3. Have you tried holding the power button down at least 10 seconds?
4. Is the sleep light at all on?
5. Have you tried tapping the shift and spacebar keys a few times?
6. Is your data backed up?
7. How old is your machine?
The OP has a MacBook Pro, so that's why the responses were specifically for a portable. The adaptor ring refers to the power adaptor, and the sleep light is a light that pulses when the iMac is asleep. I don't know if your model has one, but I know earlier iMacs did.
Here is an article for an iMac:
I'm not an expert on iMacs, but one thing to consider with a machine of that vintage is that the hard drive may have died, in which case you would expect to see a flashing question mark when you boot up. It's also possible the RAM may need to be reseated.
Is there any sign of life at all? If not, it could be that the Mac is not getting power for some reason. If there are signs of life, then it's a matter oif troubleshooting.
You happened to land in the MacBook Pro forum. There is an iMac forum, and you would have a better chance for help if you went there and posted a new question. That's where you will find the iMac experts.
https://discussions.apple.com/message/21228953#21228953 was the first thread you started, and someone did give you relevant non-portable device advice there. Please focus your comments there, instead of trying to bring almost two year old threads from the grave. You aren't as likely to get any positive feedback there.