Acer display problem

Acer Aspire 3680 laptop was dropped - the LCD has a leak but boots up ok...it displays fine on an external monitor. I bought a replacement LCD on eBay (supposed to be brand new)..it lights up but there is no display showing on the new screen - everything is still visible on the external monitor. Tried another video cable but that didn't help. I put the old LCD back in and can see Windows (Vista Home) in the background......so what else could be wrong? The mobo has onboard video but if all is well on an external monitor wouldn't this indicate that the video chip is ok? (The drivers are all up to date). Thanks in advance for any help.