If your getting a VGA signal on an external monitor then it's safe to assume your video card is ok. Because this is a fault generated by an impact, I'd suggest removing the casing around the panel 1st and checking at the ribbon connectors are seated correctly and that none have be torn away from there circuit boards. Then check the connector on the motherboard, again check for damage on the seat that the connector connects to. Reseat the cable that leads from the hinge and also check that there are no nips in the cable, it's fairly easy for an impact to damage these. If those don't work then it may need a replacement panel. Check ebay, Craig's list etc for a replacement, then if you get no joy, ring hp