id  status  mod runat
1   0   null    null
2   0   null    null
3   0   null    null
4   0   null    null

UPDATE TABLE
SET
    status = 1,
    mod = GETDATE()
OUTPUT INSERTED.id
WHERE id = (
        SELECT TOP (1) id
        FROM TABLE
        WHERE STATUS = 0
            AND NOT EXISTS(SELECT * FROM TABLE WHERE STATUS = 1)
            AND COALESCE(runat, GETDATE()) <= GETDATE()
        ORDER BY ID ASC)

1
1

1
NULL