If the characters were on retail Wrath of the Lich King, they should be on the same retail (currently Midnight) server where you left them but at a lower level (because of level squishes). If they were on Classic Wrath of the Lich King, you should be able to find them on the current Classic Mists of Pandaria servers. You will need to log in with the same email address/account to find them.
So if you played in classic WOTLK your characters are probably easy to find and you can simply download the Classic Panda client, when you log in you should see servers with a character count by them.
If you last played in original WOTLK, I suspect your characters could have been archived. In this case you would want to download the Retail Client, however if they are archived its possible you won’t see any characters, and sometimes you will need to create a new character on the servers to get them to refresh the character list. If this is the case, I would probably recommend creating a post on the Customer Service forum, the SFAs there can often see characters and give you an idea of which server your on making it a bit easier to find them. If you do post, if possible include character names, and if possible a rough time period the account would have been active.
If you played Wrath of the Litch king you had to convert to battlnet, so you may have forgotten, the best you can do is consult the customer support, and figure out what Emails you may have had back then and then use that to have your password re-set.
You only see the characters on the individual wow accounts, you don’t get access to both with a single license. The problem is it’s not really an issue the GMs work with, so I suspect if you submit a contact, more often than not they will send you a templated answer, which you might not find helpful.
Next please remove any information such as your login from your comments, as long as your logged and commenting from your Bnet account, its the character names that would be important, also nobody on the General forum can help. Try the link provided above which is the CS forum, while the SFAs there are not GMs and have limited options for doing a lot of things, one thing they have been able to do is provide some information on what server characters might be on.
By the way, you will probably want to self report your characters name, that name will likely get reported, and would likely result in an account action, self reporting it would allow you to change it and would help you avoid any penalties.