Is that a double quote or two single quotes? What syntax are you using to insert the encrypted string into the database?
Side notes:
Since I don't work with cc's I am curious... is it a good idea to actually display the full credit card number on the web page?
<cfset key = #application.key#>
<cfset Cardnum = '#Get_Orders.Credit_Card_Number#'>
<cfset CardDec = decrypt(Cardnum, key)>
This has nothing to do with the problem, but there is no need for those # signs. Just use:
<cfset Cardnum = Get_Orders.Credit_Card_Number>
<cfset CardDec = decrypt(Cardnum, application.key)>