In the U.S., gains from cryptocurrency trading are taxed just like trading stocks: as capital gains. 

Capital gains are only taxed when they are [realized](https://money.stackexchange.com/q/71121/10997). At this time, you have realized a $40,000 gain, but you have not realized a loss. If you do not sell your new position before the end of the year, you will be taxed on the $40,000 gain. (Note: your gain is $40,000, not $50,000, because you had a cost basis of $10,000 on your investment, so your gain is $50k - $10k.) Because you only held the asset for less than a year, this is a short-term gain, which means it is taxed at your regular income tax rate. (If you had waited at least a year before you sold, any gain would be a long-term gain and taxed at a lower rate.)

If you were to sell your position today for $12,000, the second trade would have a capital loss of $38,000 ($50k - $12k). When you do your 2021 taxes, assuming you have no other investment gains/losses, you would be taxed on a net gain of $2,000 ($40k - $38k). 

A very important warning:

You cannot sell your position now and then immediately buy back in and still take that capital loss on your taxes, due to the [wash sale rule](https://money.stackexchange.com/a/102174/10997). If you sell at a loss and wish to claim that loss on your 2021 taxes, you must wait a minimum of 30 days before you buy back into the cryptocurrency.