The Dell web site has good documentation for dismantling most of it's PC's & laptops and how to find and replace components.
It is unlikely that a flat or dead cmos battery would prevent bootup
On some older PC's the cmos battery is a re-chargable miniture nicad battery rather than a lithium button cell
Do you have a spare screen to plugin to check if the laptop screen has failed ?
or has diverted all output to the VGA port ?