Your answer only rephrases the question. The only way for it to hit $0 is if nobody is willing to buy the stock. This seems unlikely, given that the Bitcoin they're holding, even at 50% of present value, is still worth significantly more than $0.
Their Bitcoin holdings seems to add up to about half their current market cap, so that seems to make no sense.
EDIT: From [1]:
> “As of June 21st, 2021, MicroStrategy holds an aggregate of approximately 105,085 bitcoins, which were acquired at an aggregate purchase price of approximately $2.741 billion and an average purchase price of approximately $26,080 per Bitcoin, inclusive of fees and expenses.”
From [2]:
> Market cap: $6.08 Billion
So ~half, and average purchase price is still substantially below current BTC price.