Iphone 4 won't turn on

My iphone had a checked screen so I put a new one on and put it back together again (everything is connected properly I CHECKED!) but the phone was already completely drained of battery and before I changed the screen it was giving me the symbol that asks me to charge my phone up. But now the screen won't turn on but I can tell that it is doing something because there's a dim light on the screen like its doing something but not if you get me, but the light is still dark. Is it because the battery is fully drained or is it something else?

Try charging it first, if you've left it on charge for a while and nothings happened. Then its something else: P