What will be happen if 21 million of bitcoins become the property of one? Let's assume, that  the day already came, when emission of bitcoins completed, and we have the 21 millions of bitcoins. Let's say it happen that all these Bitcoins at one moment became the property of one person (for example because of the vulnerability of the system), which says that will not make any transaction and produces all kinds of steps to achieve this target(the deletion of the account or something else). So here is the question - what will be happen if 21 million of bitcoins become the property of one?
It's entirely up to the people living at this hypothetical future point.  There's no current rule about what happens.  Maybe when this happens, people are tired of Bitcoin, so they go back to trading seashells.  Or maybe when this happens, there's something better than Bitcoin available, so they all switch to that.  Or maybe they just restart Bitcoin and start mining again.

Remember that Bitcoin is a consensus system. Each user can change the software to do whatever he wants it to do.  If enough people all run the same software, and that software is designed to converge on a consensus block chain, all of those people will have the same block chain---even if it isn't the same block chain we use today.  Whatever rules we use today can be changed by people in the future (including our future selves), so there's no risk of ever permanently locking ourselves out of the block chain.