With Strava you can - the username is simply a display name, nothing more. When you login, you log in using an email address, not your username.
I believe you
@Adam4868 have identified the problem. You have 2 accounts. Therefore the system thinks you are different people. You need to log in using the email address of the account that has the data in it, not the new one. If you can't remember the password for the old account, then do a password reset on the other email address, the one with the data. No need to involve Strava support which will take about 48hrs to respond. And your data is not gone if other people can still see you. It is simply a case of working out which account email address you need to log in with. So signing in with a FB account or not or working out which email address you were logging in with. Just keep trying the combinations until you hit the correct one.
Also remember that she may have fallen for the sign up page which it always presents first rather than the little login option you have to go and hunt for...