Sounds like that last BIOS flash bricked-it (killed the motherboard).

Both the event and possible fixes have been written about many times here. As a last resort, you can try removing the CMOS battery for a few hours.

Unfortunately, it boils down to this:

Here is the Dell BIOS recovery options.  If you are lucky one of the procedures will work.

https://www.dell.com/support/article/us/en/19/sln300716/bios-recovery-options-on-a-dell-pc-or-tablet...

Here is Dell's policy on a corrupted BIOS update:

https://www.dell.com/community/Product-and-Support-Issues/Dell-Policy-BIOS-breaks-motherboard-CMOS/m...