There are two different ways to report a loss from stolen cryptocurrency. Based on how you use Crypto Currency will determine the tax treatment.
In Canada, the CRA expects all Crypto-Currency transactions to be treated in the same manner as any commodity would, which means any increase in the price produces a Capital Gain (taxable at 50%), and any losses would create a Capital Loss although;
If you are considered a high-volume trader the treatment would be different. A high-volume trader is someone who holds Cryptocurrency for a short period of time before trading them. In most cases, the CRA will consider this an adventure in business meaning you’ll have to file your taxes as such.
Most likely if you are not actively buying and selling you would report your loss as a capital loss which would be its book value or adjusted cost base. In order to claim this you would need to go to your tax return and declare the capital loss under the "Capital Gains and Capital Losses Deduction Page"