Ideally all your purchases should be mde with one iTunes account/Apple ID. Even if the login email address has changed or no longer exists you can still login using that information, The contact email address should be kept up to date in the account configuration information.
If you have set up multiple accounts then life will be messy because you cannot transfer content to other accounts and you will have to manage multiple accounts.
If you have lost information you can ask Apple to help reset information but you will have to be patient because they will need to know it is really you.
iTunes Customer Service Contact - http://www.apple.com/support/itunes/contact.html > Get iTunes support via Express Lane > iTunes > iTunes Store
You can find out which account was used to purchase an item by getting information on that item in iTunes and displaying summary information.
Thanks very much for this informat, Limnos. Yeah, moniker was forced to change : local provider allowed three character name, moved and so moved to comcast, which required more than three. Moved to gmail, kept same >3 char moniker. Not sure why this may have changed our account, but it is asking for passwd from original isp.