What model Dell computer? The function of FN + F2-12 can be toggled to change the default setting. 

XPS 13 Plus 9320

The function buttons are not lit up and i am unable to press them

What BIOS revision are you on?

v 1.7.1

Sort of unrelated but the 1.7.1 revision does have issues other than yours. But anyways, have you tried loading defaults in the BIOS? If not, boot into the BIOS by holding the shift key when you click restart inside of Windows. It will take you to a recovery menu where you can select UEFI options and then restore BIOS defaults. Also does the "touch bar" light up at all (for instance on boot up even briefly?)

I have tried loading BIOS defaults however that doesn't seem to work and the touch bar does not light up at all when booting

Sounds like it could be a hardware failure, you can try the Dell Diagnostics that are in the BIOS. Also reach out to Dell though (if they do not reply to this forum) and see if you can get a replace/repair if the troubleshooting does not resolve the problem.

I have exactly the same problem with a XPS 9320 laptop. I do not know why the funcion bar stopped working, but it is annoying. 

@Luis Sanchez323eee​ I have exactly the same today after a random software update happened

Same issue today.  No idea why this is happening. VERY DIFFICULT to work witout, delete, insert etc as well as function keys.  XPS 16 9640 

Processor         Intel(R) Core(TM) Ultra 9 185H (2.50 GHz)