It’s hard to say what happened but I would start with a simple test; disconnect the LCD and Digitizer (battery first!) and then reconnect the battery and connect the iPad to an iTunes enabled computer. What this does is essentially connect only the logic board to the computer. If you wanted to take this a step further, disconnect the cameras and buttons as well. If the logic board is still not recognized, then you most likely have a battery or logic board issue.
What was the condition of the battery prior to this? A weak battery would cause similar symptoms and this could be a coincidence (you are a shop I presume so you wouldn’t necessarily know the history of the battery). Measure the voltage of the battery with a multimeter and insure it is properly charged. If it is, then the problem will be with the logic board. Depending on your level of skill, you could start probing the board. At the very least, do a thorough visual inspection of the board, particularly in the connector area and the small components that surround the connectors.