Hi! so, to recap: laptop powers up, screen's on too (and you were even able to run the LCD built-in test, so we can assume that it's both connected and in good working order), but you see no Dell logo... Windows is not booting. Since the LCD test works, I would assume that the BIOS or at least the ePSA is still working, which begs the question: are the status LEDs blinking in a particular pattern, or not at all? Furthermore: when you turn on your laptop, if you press F2 you are not able to enter the BIOS, are you? Same with F12 to enter pre-boot options? You have already replaced the CMOS battery, so we can safely assume that BIOS settings have been reset. If you haven't tried this already, you could unplug the AC adapter, remove the main battery, press the on/off button for at little more than 30 seconds, and then replug the charger (not the battery) and see if it makes any change at all. If this doesnät work, and we can't reach F12, we can't run any form of diagnostic test on the hardware, we can't even try to recover the BIOS. All you can do is remove unnecessary hardware piece by piece, starting from RAM sticks and even the video card (this will forcefully enable the iGPU). You should try different video outputs as well, especially the dear old vga. Check the hard drive as well. Still, this kind of hardware failures would be detected by the ePSA, but if you have no blinking LED and no way of reaching the preboot menu your hands are pretty much tied. It could be that the BIOS is bricked, in that case you would need to rewrite the eeprom  (or replace the failing memory if that's the case), something only a skilled technician can do... There are stores that offer free diagnosis if you send in your laptop, they can replace capacitors, fix circuits., rewrite the eeprom.. you could ask for a quote if you run out of options.  
I have a Precision M4700 workstation, bought in 2012. I hadn't used it for more than a year and a couple of weeks ago I tried to turn it on and... surprise: the screen shows nothing, it continues to be black.   I tried to do these steps:   Could you suggest me other tests, in order to understand what it happened?   Thank you
Precision M4700 black screen