I don't know what you mean by "loss."
When you listed the car as an asset, you should have been taking depreciation all along.
If you choose to use the actual expense method, you can claim your repairs as a business expense. If you use the mileage rate, you can't deduct your repair costs in addition to the mileage.
When you disposed of the car, treat it as a sale with a selling price of zero (unless the junkyard gave you something). Assuming the car is properly listed as an asset in Turbotax, it will be handled correctly.
You don't get any extra "write off" -- that would be a double deduction. You've already taken depreciation, you've already deducted your repair expenses. You don't get anything extra for the fact that you have lost the possible future use of the car.
