If I understand correctly, you were updating your S2 with the official update and during the procedure your battery went off. What happens if you plug the charger into the phone, do you get any charging icons or signs?

Try one of the following options:

You probably already did put the new battery out and back in, but do this once more, just to be sure.

Hold the power on button and plug the charger in and out for at least 6 times, as you are still holding the button. I know it sounds weird, but this actually helped a few people.

Hard reset of Galaxy S2 i9100

But to warn you, this will wipe all your phones data, like contacts, images, messages, apps,...

1. Press and hold Volume Up + Home Button + Power like this:

Press and hold Volume UP and Home button then press the Power button for 2-3 seconds while you are still holding Volume up and Home

2. Release only power button, but keep pressing Volume up + Home

3. Using Volume down select wipe Data/Factory Reset

4. Press Power

1. Hold the Volume Down button

2. Press and release the Power button

3. Select Clear Storage using the Volume Down button

4. Press and release the Power button

5. Press Volume Up to confirm i9100 master reset

If non of the options above will work for you, then it's a great chance of a broken motherboard or any other vital components of the phone. I hope you fix your Galaxy S2, let me know, how it went.