Convert Varchar To Date In Where Condition

Hi guys.
i have a field in a table which datatype is varchar and i m storing datetime and number in it.. but when i try to use select statement like ( between startdate and enddate) or number (= ) operator it's not working..

select * from table
where cast(Field as datetime) between @startdate and @enddate



second query that i m trying to exectue is

select * from table
where cast(Field as bigint) = @Fieldid


but both are not returning any value...can anyone tells me where i m doing mistake