Not enough information. Could be missing indexes, could be incorrect stats, could be a few other things

Please post table definitions, index definitions and execution plan, see http://www.sqlservercentral.com/articles/SQLServerCentral/66909/ for details

Cleaned up query:

SELECT COUNT(*) FRMP_RecordCount ,

b.RECORD_ID FRMP_Record_Id ,

b.METER_ID FRMP_Meter_Id ,

CONVERT(VARCHAR(10), a.FromDate, 103) FRMP_FromDate ,

CONVERT(VARCHAR(10), a.ToDate, 103) FRMP_ToDate ,

CONVERT(VARCHAR(10), b.Billing_Start_Date, 103) FRMP_Billing_Start_Date ,

CONVERT(VARCHAR(10), b.Billing_End_Date, 103) FRMP_Billing_End_Date ,

NetAmount FRMP_NetAmount ,

a.Party FRMP_Party ,

b.PROVIDER FRMP_Provider

FROM ( SELECT d.RECORD_ID ,

d.METER_ID ,

d.BILLING_START_DATE ,

d.BILLING_END_DATE ,

d.NET_AMOUNT AS NetAmount ,

d.PROVIDER

FROM ( SELECT METER_ID ,

BILLING_START_DATE ,

BILLING_END_DATE ,

PROVIDER

FROM NETWORK_VALIDATION..evd_NonNBV_ForChecking_BAU

GROUP BY METER_ID ,

BILLING_START_DATE ,

BILLING_END_DATE ,

Provider

HAVING SUM(net_AMOUNT) <> 0

) c

INNER JOIN NETWORK_VALIDATION..evd_NonNBV_ForChecking_BAU d ON c.METER_ID = d.METER_ID

AND c.BILLING_START_DATE = d.BILLING_START_DATE

AND c.BILLING_END_DATE = d.BILLING_END_DATE

) b --From #MyTemp_1 b

LEFT JOIN NETWORK_VALIDATION..MSATS_DATA_BAU_FINAL a ON a.NMI = b.METER_ID

AND ( ( ( b.BILLING_START_DATE BETWEEN a.FromDate AND a.ToDate )

AND ( b.billing_end_date BETWEEN a.FromDate AND a.ToDate )

)

AND a.Role = 'FRMP'

)

--Where a.NMI Is Null

GROUP BY b.RECORD_ID ,

b.METER_ID ,

CONVERT(VARCHAR(10), a.FromDate, 103) ,

CONVERT(VARCHAR(10), a.ToDate, 103) ,

CONVERT(VARCHAR(10), b.Billing_Start_Date, 103) ,

CONVERT(VARCHAR(10), a.ToDate, 103) ,

CONVERT(VARCHAR(10), b.Billing_End_Date, 103) ,

NetAmount ,

a.Party ,

b.PROVIDER