|I assume you never see anything on the screen.|
Many possibilities but wouldn't hurt to clean edge connectors of RAM with pencil eraser then pop the sticks in and out a few times to clear any oxide off the sockets too. It's a fairly common cause of issues.
Also try removing the power (and battery if it's a laptop) then holding power off/on button for about 20 secs. Probably won't help but quick to try.
Always pop back and let us know the outcome - thanks