Laptop won't boot up

My laptop won't boot up. I accidently left it on and the battery died. After that, it stopped working. I tried hard resetting but it didn't work. The laptop charges and the number and cap lock light is on when I turn on the laptop. The fan is working and everything but I have to plug in the ac adapter for the laptop to turn on. I don't know what I should do and I don'the want to buy a new motherboard.