How to reset bill no everyday in SQL or C#

If you would like to increase BillNo till next date in multi-user environ , i'd suggest to create computed column in your table, which will get SQL function as a formula. This function will get a date of sale as an input and return MAX(BillNo) +1. For example:

CREATE FUNCTION BillNumberForDate(@inputDate DATE TIME) RETURNS INT
AS
BEGIN
    DECLARE @bn INT = 0;
    SELECT @bn=COALESCE(MAX(BillNo), 0) + 1
    FROM YourTable
    WHERE CONVERT(DATE, SaleDate) = @inputDate
RETURN @bn
END;

How to add this function to computed column in your table? See: Formula For Computed Column Specification Property in SQL Server 2012[^]

For further details, please see:
CREATE FUNCTION (Transact-SQL) | Microsoft Docs[^]
Specify Computed Columns in a Table | Microsoft Docs[^]

Note:
I'd strongly advise you against to "produce" BillNo on client side for set of reason. For example: you'll get duplicated BillNos.