I am not certain about your meaning of "lost rent." Did someone default on a lease, leaving a unit that you own vacant or failed to pay rent and you could not recover it as part of an eviction action? In either of those cases, if you are a cash method taxpayer, there is no deduction, because the rent never was counted in your income.
If, as is less likely, you use accrual method accounting and had already accrued rent into income, the uncollectible amount would be deducted as a business bad debt.