As some people have already suggested, first try switching on wifi, if that doesn't work, try changing the date and time manually to the correct ones as it will likely have set itself to Jan 1970 or something similar when you took the battery out.
If neither works, also try checking the cellular antenna and that it's in the right place (NB. the pics in most of the guides on here actually have it in the wrong place - it needs to go around the left of the standoff then to the right of the metal finger)