Bios 2.5.1 was sufficiently old enough I was able to get back in. I disabled secure boot, pressed F12 to get to boot options, flashed BIOS to 2.5.1 (which I had saved to a usb), re-enabled secure boot, logged into my PC, and printed the bitlocker recovery key.   I then rebooted computer.  It immediately reinstalled the BIOS update that originally caused the problem, but I had a printed recovery key.  Yay!
OK, I never set up bitlocker on this laptop, and I never set up a Microsoft account... I only set up a local login.  An automatic update kicked off yesterday.  It didn't really ask me, it just did it.   Now I can't log into my laptop, because it comes up to a bitlocker recovery screen.  I don't have the key, and there is no key saved anywhere, since I never set up a Microsoft account and never did anything with Bitlocker. In other threads here on the Dell Community, I saw that some other people have had luck with reinstalling the previous Bios and then getting back into the laptop. However, when I look at notes with the last BIOS update, it says the following: Important Information - Once the BIOS is upgraded to version 2.5.1 or later, you cannot downgrade the BIOS to version 2.4.2 or earlier. BIOS downgrades are restricted when there are security updates and important fixes with earlier versions that impact the functionality of the system. Am I even going to be allowed to downgrade back to what I had before? Any other advice on how to get back into my files?  I'd rather not lose all of my data.  
XPS 9360 Update Triggers Bitlocker screen