Reading your question through again I'm guessing that you actually have the bank name as the selectedvalue in the ComboBox and not the bankid.
If you have the bank name and want to get the cheques based on that, then you just have to join to the bank details table ...
SELECT BC.bank_Id,row_number() over(order by [bank_Id]) as SlNo,[bank_Chqstartno],[bank_Chqendno],
case bank_Chqlvscount when 100 then 2 when 50 then 1 when 10 then 0 end as noofcheques ,
bank_Chqlvscount,bank_Stat
FROM [dbo].bankcheques BC
INNER JOIN [dbo].bankdetails BD ON BC.bank_Id = BD.bank_Id
where bank_name=@bank_name
order by SlNo desc