my dell latitude e6420 screen goes blank when it has to boot

Hello D. Slim,

Does it happen on and off of the docking station?
Do you have known good hard drive that you can swap in?  If not, are you able to reseat your hard drive?  Sometimes, the hard drive will work itself loose and cause problems such as this.
Do you have known good memory that you can swap in?  If not, are you able to test one stick of memory at a time in the system?  There may be one bad stick of memory that is causing the problem, or one bad memory socket on the motherboard.  Here is a link to a Youtube video on how to replace the memory and hard drive on your system:  http://www.youtube.com/watch?v=k7Zvp-XSfzk 
Are you able to get to the Bios or run diags?  How about the diagnostic LEDs?  Is the power button solid white, amber, slowly increasing/decreasing in brightness, or off? How about the storage, power and wireless LEDs?  Are they solid, blinking or off? 
When did this start?  Is this a brand new machine?
Is the system configured to PXE boot?  If so, is there a PXE server available?  Updating to the A12 version of the Bios should resolve the PXE boot issue if it is related to this occurance.
Please let me know what you discover so we can isolate what is happening on your system.
~Maureen
#iwork4Dell

Hi D.Slim, I was checking back in on this case for Maureen. Have you had a chance to try any of the troubleshooting steps she had mentioned. Please let us know if there is anything else we can do at this time. -saul #iwork4Dell