You might have seen that there is an option to log in with Email and Password. This register type existed only for a while, so if you didn't register an account with plain Email / Password combination, you won't be able to login with such option.
To check, whether you were login or not, tap "Forgot password" → Enter email → Send. If there would be no record with such email - there would be a message about it.
If you are sure that you were login in the app previously, try other options, one by one (Google / Facebook / iCloud / IMAP / TripIt). If you log in with the right account, you will see that your flights would start appearing in the app.
In case any questions, just write us at firstname.lastname@example.org