Iphones and Ipods are extremely delicate! Although things look easy following guides on how to change the screen etc... When you actually start doing the work the slightest slip can ruin the whole device.
Because of this there could be any number of things wrong with it and its an aladdins cave to start hypothesising without further info
Its possible you have dislodged the antenna or damaged the cable to it and the only way of repairing if you are lucky is to replace that part. Or that a part of the mainboard has been shorted or it could be as simple as reactivating the phone with Apple via SAMprefs but to do that you need the device to be jailbroken. Like I say it could be any one of a million things.
What exactly did you do with it and were there any slipups that you made?