Characters are specific to the account/subscription that you logged in with and the server that was logged into when the character was created. The two most common reasons for not finding a character is you are either logging in with the wrong account/subscription or you are logging into the wrong server. Contact Account Management and they can help find any/all accounts/subscriptions you have. If you know the character name, they can find what server they are on.
If, on the off chance, your account was stolen, Account Management can help with that as well. If the account was stolen and the characters deleted, there is a process to follow to have them recovered (free of charge). See the sticky in the Account Support Questions forum (bottom of forum list) on what to do if the account is stolen.
"Everyone's got one. Better to be a smart one than a dumb one." - me
"It's people like you that [I]think[/I] you know everything that give those of us that [I]do[/I] a bad name." - me
"Eat, drink, and be Merry, 'cause Pippin was a fool of a Took."