|I assume you mean the screen is blank and that you get a long continuous sound. |
We don't know your computer spec so it's a bit of a guess, but this usually indicates a hardware fault.
One simple thing you could try is to clean the RAM edge connectors with a pencil eraser, then pop the RAM sticks in and out a few times to clear any oxide off the sockets themselves.
Ensure all connectors inside are push firmly home.
Always pop back and let us know the outcome - thanks