|It's EASY to find manuals for most Dell computers - you just search the web for: (name of model) documents|
and one of the "hits" on the first page of results will probably be for the manuals the Dell web site has for it.
Manuals - English
Dell™ Latitude™ E6420
See the SERVICE manual.
Remove the battery and the AC adapter's connection when you're fiddling with the keyboard's ribbon cable.
You have to be very careful when you unplug or plug in the ribbon cable for the keyboard - it's easily damaged. The contacts on it's end must face the right way when you plug it into it's socket. The ribbon cable and the socket you plug it into may have contacts on one side only.
If the ribbon cable and the socket have contacs on both sides, if you have plugged it in backwards then tried booting the computer, you have probably fried the keyboard's circuitry.
In most cases, unless your model is really old, you can buy new clone keyboard on the web for a reasonable price, but you must get one that's a match for the same Dell part number - that's often on a label on the bottom of the keyboard.
In the mean time, you can use a USB corded keyboard for most things