Plug it off, remove the battery, press and hold down the power button for a minute or so, put back the battery, plug it back in, turn it on and see if it starts up.
If that doesn't work, remove the memory card (usually it should be easy to access and remove without taking apart the entire laptop) and put it back in.
If that doesn't work and you have more than one memory card, try only putting in one of the two.
If that doesn't work and you have an external monitor, connect your laptop to it and see if any image shows up on the monitor. If it does, something is wrong with your inverter or LCD flex cable. If that's the case, you need to take it apart, check to see if any cables connecting to the screen are disconnected or cut. You can either connect them yourself or get it repaired by someone else.