If your battery has run completely down and it been sitting for a while it may not be retains enough power in the capacitor on the logic board to remember who it is, what wi-fi network it is on or even the date. First thing to do is what you know is wrong with it. Replace the battery and let it fully charge up. Now you can try to power on and see if it will talk to you.
If that fails, disconnect the battery and the mag-safe, then press and hold the power button for 10 seconds. Now reconnect the battery and mag-safe and it should reboot spontaneously.