Hi,
Try a power refresh in case the BIOS has glitched and is only connecting to the external monitor.
Turn off the laptop if on and disconnect the charger (if connected).
Disconnect the main battery from the motherboard. You don’t have to remove it, just unplug it.
Remove the coin cell battery from the motherboard. Note the orientation of the battery before removing it, so that you will reinsert it the correct way later. Usually it is +ve on top.
Here’s the [https://dl.dell.com/topicspdf/latitude-14-7400-laptop_owners-manual_en-us.pdf|service manual] for the laptop. Go to p.20 and p.22 to view the necessary pre-requisite steps and then the procedures to disconnect the main battery and to remove the coin cell battery
Press and hold the laptop’s Power On button for a full 15 seconds to reset the BIOS back to factory default, then release it.
Reinsert the coin cell battery.
Reconnect the main battery and reassemble the laptop.
Connect the charger and turn on the laptop and check if it is OK.
If it is then there may be a message regarding the date and time being incorrect. This is normal as the BIOS has been reset. Once the date and time has been corrected the message won’t show again the next time that the laptop is started.
Also any user defined settings that were altered in BIOS to suit your particular operating requirements will have to be set again.