MacBook Pro won't boot I have a Macbook Pro 8.2, Intel i7, 4 GB of RAM and 500 GB HDD.

While working with some apps open in background, the screen suddenly went blank and music looped. In panic I pushed the power button to shut the Mac down. After a while, I tried to boot. It paused half way and the screen turned to grey but didn't shut down for half hour. I decided to turn it off.

I was searching and found some troubleshoot for my problems. I tried to reset PRAM and SMC. I tried to replace Battery, RAM and HDD, then reset PRAM and SMC. Neither worked.

I tried to boot to Recovery Mode, but the Recovery HD can't be found on my HDD. Then I tired to boot to Internet Recovery Mode and after download finished, it turned grey for half hour or more and finally shut down.

I tried to boot from DVD by holding C which wasn't working. On startup manager, I found my primary startup disk that failed to boot, but it can't find the disk.

Please help and guide me to backup my files and install clean os x offline or online.
Sounds like a bad RAM slot.  I had a similar issue with that and garbled screen.  It's still possible to run it with a single stick of memory if one slot is bad.