How you treat lost or stolen coins depends on your local regulations. Usually, there are two methods you can use (please check which is correct in your jurisdiction):
Method 1: Sell the lost asset for 0 and realize a loss
For example, say you lost 1 ETH. In this method, you would enter this as a "Sell" transaction with the price of Ethereum being 0. This would make your proceeds from the sale 0.00 which would lead to a loss (if you paid more than 0.00 for that Ethereum).
Method 2: Sell the lost asset at the current market value
Again, say you lost 1 ETH. In this method, you would enter this as a "Sell" transaction and set the price of the ETH to the market value of Ethereum on the date the asset was lost/stolen.
PRO TIP: Add that the asset was lost/stolen in the description of the transaction for future reference (optional).
PRO TIP 2: For method 2, if you do not know the market value at the time the asset was lost/stolen, enter the exchange, pair, and date first. The market value of that asset will autofill based on the history we have for that asset. You can customize this as you wish (check the price is accurate for your method).