Ghosted after 4 rounds?

I had a very positive interview experience with one of the firms. I had cleared 3 rounds and my final round was a culture round with HR head and few colleagues from other departments.

We had a very fruitful and fun conversation as well throughout the call.

But after that it's been 3 weeks and they haven't contacted me or even sent a rejection mail. Not sure how to go about this, I have sent the HR a mail too to check on any updates. Any advice on how should I proceed?