Help! my screen is blank at boot up

My laptop is Dell Latitude e6420. So I was using my laptop connected to an external monitor since my laptop's lcd is cracked. I decided to replace it and followed the steps. Luckily, it was successfully installed. When I tried it out, there was no logo of dell nor the windows 7 logo but the screen will only show up when I'm at the welcome screen. the boot up screen is only blank even when i go to bios menu or setup. When I tried connecting it aain to an external monitor and tried to enter the bios menu and press fn+f8 to switch it to external monitor, the bios menu worked. my only problem is, when im using the laptop's LCD, the boot screen would be blank (no dell logo and windows 7) but the screen would work only at the welcome screen and work perfectly til then. how can I fix this?