If you are using Sql Server this should work
DECLARE @Table TABLE(
ID INT,
Val INT
)
INSERT INTO @Table (ID,Val) SELECT 1, 3
INSERT INTO @Table (ID,Val) SELECT 2, NULL
INSERT INTO @Table (ID,Val) SELECT 3, 5
INSERT INTO @Table (ID,Val) SELECT 4, NULL
INSERT INTO @Table (ID,Val) SELECT 5, NULL
INSERT INTO @Table (ID,Val) SELECT 6, 2
SELECT *,
ISNULL(Val, (SELECT TOP 1 Val FROM @Table WHERE ID < t.ID AND Val IS NOT NULL ORDER BY ID DESC))
FROM @Table t