Lost & Stolen Crypto
Losing the private key to a wallet containing your cryptoassets cannot be treated as a disposal for capital gains tax purposes. The private key still exists as part of the cryptography, albeit it is not known to the owner any more. Similarly the tokens will still exist in the distributed ledger.
Being the victim of theft resulting in the loss of cryptoassets is also not considered a disposal for CGT purposes, as the individual still owns the stolen asset and has a right to recover it.