You might have disconnected the connector of the screen when you opened it. This connector usually snaps back when the screen is in a certain angle when being opened.
For the location of the connector, see step 5 here; it is the connector on the right, bottom side right below the camera lens.
In order to reconnect it, try this: While holding the screen open in a 45 degree angle, use an iPod plastic opening tool (see step 2 of the same guide mentioned above), position the connector on its socket, and - via the plastic tool - push it carefully back into it. You need to hear a click since it snaps in.