I have a Samsung Q45 running MS Vista home. The laptop displays a black screen upon boot up. There is a flick of light very brief and then black screen no curser. If left to run I can hear the windows start up tune. I have tried connecting to an external display but the display is black screen also. I have removed the HDD and virus scanned it still no joy. I have changed the hdd cable to the motherboard. I have tried booting with and without the battery. When i removed the hdd i found it still works. The windows start up tune suggests that too works. I can not get to the BIOS screen either and have tried leaning the battery touch points.

Since you have already taken it apart, open it back up remove the RAM, HDD, and CMOS BATTERY. Leave that battery out for a few minutes. Also while you have it apart, see if you can remove the CPU/GPU cooling fan, they always clog up with lent. Your problem sounds like maybe a faulty GPU. If you can hear windows boot an play music then your HDD and computer are working but you can't get VIDEO output. When hooking to external monitor did you push the FUNCTION KEY and the assigned F1 - F12 key to toggle the exterior VGA port on? Give these a try and comment back if you need more instruction. Hope this helps.

Your screen-cables might became lose.

Contact your local computer-repair shop to fix this.