An asset that is fully depreciated and continues to be used in the business will be reported on the balance sheet at its cost along with its accumulated depreciation. There will be no depreciation expense recorded after the asset is fully depreciated. No entry is required until the asset is disposed of through retirement, sale, salvage, etc.
To illustrate this, let's assume that a machine with a cost of $100,000 was expected to have a useful life of five years and no salvage value. The company depreciated the asset at the rate of $20,000 per year for five years. If the machine is used for three more years, the depreciation expense will be $0 in each of those three years. During those three years, the balance sheet will report its cost of $100,000 and its accumulated depreciation of $100,000 for a book value of $0.