If you replaced your iPod/iPhone screen and now your devise won't power on you may need to ground the battery. Here is a little tutorial on how to do it. I have successfully done this on several iPods.
Circled here is the battery terminal you want to ground. The rectangle metal part shown on the body is what you want to ground it against. **Make sure the iDevice is unplugged**
Take a small screwdriver & touch the battery terminal and metal housing at the same time to ground. You may see small sparks and this is fine.
Now plug in the power supply & you will see the Apple boot logo, (NOTE: Make sure both of the screen connectors are plugged in the back of the iDevlce and that no cables were torn)