[NAME]  Newer versions of Dell Update default to suspending BitLocker when you choose to install a BIOS update, specifically to avoid this situation.  Maybe you're running an outdated version or unchecked that option?  In any case, what's happening here is that normally, the TPM releases the decryption key for BitLocker -- but it will only do that after a successful "platform integrity check" that verifies that certain hardware and firmware aspects of the environment have not changed since the trusted state that the key was "sealed" against, since some changes could be part of an attempt to compromise the key when the TPM releases it.  On some systems, a BIOS version change counts as a change that breaks that platform integrity check.  At that point, the TPM won't release the key, and you're prompted for the Recovery Key instead.  If you enter it, the TPM re-seals to the new state.  (Alternatively, if BitLocker is suspended at the time of the change that normally would cause the platform integrity check to fail, then the TPM reseals against the new state automatically and you don't have to deal with a Recovery Key prompt.)So ideally you'd have the key backed up somewhere, possibly in the cloud under some Microsoft account you've used with that laptop.  If you can't find that, then the only other way to solve this would be to use the BIOS update utility built into the firmware to revert to whatever BIOS version you were previously running, which will return the system to its trusted state and allow the TPM to release the key -- assuming you haven't gone ahead and cleared the TPM to try to troubleshoot this, in which case the key will be gone.  To use the built-in BIOS update utility, press F12 during startup and choose "BIOS Flash Utility" in the one-time boot menu.  That will take you to an interface where you can specify a flash drive that contains a BIOS update EXE file.  You can find those files on the Dell Support site.  Enter your model or Service Tag, go to the Downloads section, and search BIOS.  Click the appropriate search result and select "View older versions".  If you don't remember which version you were running before, you'll have to try a few.  But download those EXEs and put them on the flash drive to use with the utility.And whenever you get out of the woods, back up your BitLocker Recovery Key.  There are times you will need it and won't have another option like this, such as after a motherboard replacement where you'll have a new TPM that won't have the decryption key anymore.  That is another situation that has caught several people out.
My laptop xps13  got a BIos software update by Dell yesterday. On restart my laptop was Bitlocked.  I desperately need to get my laptop up and running again. Please help
XPS 13 Bitlocked after Bios update yesterday