XPS 15 Start Failure (possibly after Windows Update)

Hi,

my Dell XPS15 9500 from 2020 (Intel i7, 10th Gen, originally 16 TB Ram, I upgraded last year to 64 GB RAM) fails at starting Windows (11). Yesterday when I turned it on, I was unexpectedly asked for my Microsoft Bitlocker Recovery Key (which I had never used before, but which I found in my Microsoft Account). I entered it, and afterwards the laptop could start (twice), but while displaying a bright coloured bar at the top of the display for a couple of seconds it completely froze and I had to force-shut it down. Now when I start it, only the Dell Logo will appear, and the power LED will blink in a repetitive pattern (4 times white, 2 times orange). 

I already tried a cold start  (deconnecting the battery, pressing the power button for 40s and reconnecting it), nothing changed. I thought maybe it is due to the changed RAM, so I put in my old 8GB Ram bars, same thing. I cannot enter Safe mode (F4/F5) nor enter the BIOS (F12), as if the keyboard does not work anymore. 

I have all my data backed-up, so I would be fine with resetting Windows completely, but without entering the BIOS I am not sure how to do so. Anyone have any idea on this?

Maybe unrelated, but I have always had some issues with the Windows Updates, the laptop would turn on by itself at night with the fans running wild. Rarely also the laptop would freeze, which could be undone just by putting it to sleep mode and awaken it again. Apart of that it worked fine for the last 3 years.