Checkout Not Working After Re-Uploading "Checkout" Folder

So after running into a Syntax Error during the checkout process (on the Shipping Step), I re-uploaded the "Checkout" folder. NOW.....the checkout process doesn't work at all AND clicking on "add product" no longer works.

What gives!?! I don't understand WTH happened.....can anyone help?

Thanks!

I'm running 1.5.1.3 by the way.