Why is my computer not turning on?

I've been having problems with my computer for the past few days, It's a Lenovo (not sure what model though). When I go to turn it on, It doesn't. When my computer is dead, there would be a red flashing light from the bottom of the computer, it's suddenly not on. I've tooken the battery out for a while then put it back in, Still nothing. I charged it for two days, Nothing. The last time it did this it randomly turned back on after a few hours. But I would love to know why it's doing this, Thanks!