I personally owned a E5420 with this problem.
I can tell you none of the suggestions here work.
Sure you have your Serial Number, Model Number, Express code, etc.
Calling Dell with these numbers will not help unless the laptop is still under a service contract or extended warranty.
Calling Dell without these will get you literally laughed at (you can hear them in the background) and they will ask you for about $120USD to provide the proper code.
The Latitude E5400 is an Enterprise class laptop, hence incredible layers of security.
Pulling the CMOS batter will not help.
Removing the battery and holding the power button will not reset it.
Replacing the CMOS chip itself is overkill.
What you need to do is note the CMOS code it provides you and find the proper unlock on Ebay and pay them for the master unlock code, which I ended up relenting to do.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p3984.m570.l1313.TR0.TRC0.H0.Xdell+latitude+bios+removal.TRS0&_nkw=dell+latitude+bios+removal&_sacat=0