Regardless of whether you lose them you pay taxes when you make a taxable transaction, being either exchanging FIAT to crypto or crypto to a different crypto.
So if you mined and then traded $100k in Bitcoin for Ethereum and then the Ethereum was stolen, you would use form 4684 to claim the loss against your taxable income.
But I haven't heard of anyone doing this yet and I can imagine it would get some scrutiny from the IRS, seeing as it would be hard to prove a loss or theft.