The explanation post from leafo, while an explanation for erratic behaviour and ups and downs, is not a sufficient explanation why the game of OP was suddenly sorted last in the browse of popular games. (It only made clear, that it is not something to do with the account)
I did eventually find our game at the very very bottom of the popular page (took 5 minutes to scroll to the VERY bottom)
Something happened. And it is prudent to assume it affected more than one game. And to happen in such a way, to include it, but sort it last, the sortin algorithm must have thought it to have near 0 popularity. Even when allowing for generous adjustments to mix in boosts for new games, this is too inaccurate to be considered normal behaviour. Either the data was wrong or the data processing was faulty.