IF EXISTS (SELECT 1 FROM SYSOBJECTS WHERE name = 'tba')
BEGIN
    DROP TABLE tba
END
GO
CREATE TABLE tba
(
    StartTime VARCHAR(7),
    EndTime VARCHAR(7)
)
GO
INSERT INTO tba
SELECT '08:20','16:00' UNION
SELECT '16:01','20:00' UNION
SELECT '20:01','4:0'
GO

DECLARE @Current VARCHAR(7)
SET @Current = '21:00'

SELECT * FROM tba
WHERE @Current BETWEEN StartTime AND CASE WHEN LEFT(EndTime,CHARINDEX(':',EndTime) - 1) < LEFT(StartTime,CHARINDEX(':',StartTime) - 1) THEN RTRIM(CAST(LEFT(EndTime,CHARINDEX(':',EndTime) - 1) AS INT) + 24) + RIGHT(EndTime,CHARINDEX(':',EndTime))
                                     ELSE EndTime END

StartTime    EndTime
20:01    4:0