Why does my laptop keep shutting down?!?!?!?

I NEED HELP DESPERATELY! My laptop keeps shutting down and i don't know what the problem is! I cleaned out the fan to get all the dust out, but it still overheats. So i guessed the problem was the battery underneath had to be removed. So i bought a battery for under the laptop and got it in the mail today. That turns out to not be the problem. I don't know what else to do! I don't know why it overheats still. Could you please tell me why it overheats still and give me a solution? THANKYOU!!!!!!!!!!