Hi Jade
As you say, first off you need to make sure no depreciation has been journalled in since the theft.
Next i'd create a new nominal code in the P&L called something like 'profit/loss on asset theft'.
You'll then need to do a journal as follows:
CR Fixed assets for the original purchase price
DR Accumulated depreciation for the depreciation to date
DR/CR the balancing amount to the new P&L a/c called profit/loss on asset theft
Are we talking a reasonable sum here? If so, make sure you keep supporting evidence in the form of police report docs etc.
If there's an insurance claim the sum received will go back into the accounts as other income or something along those lines.
Then, if you re-purchase said asset it'll be capitalised in the usual way.
Come year-end make sure your accountant is aware of the issue as the theft itself won't be allowable for tax purposes [as presumably the purchase has already been claimed as a capital allowance].